Frank Sinatra plays a great killer/ psycho in
SUDDENLY. CHECK IT OUT

A shock to the system (1990)with Michael Caine and Unleashed (2005) with Jet Li.

If you are at all partial to film noir, I would suggest This Gun for Hire (1942). It's a great movie.

This was the last great statement in 20th Century Hong Kong Cinema's Female Assassin subgenre. It's philosophically deep, said philosophy being Confucianism. The story is also firmly rooted in it's contemporary political setting of 1996 and the anxiety over the imminent handover of Hong Kong to the Maiinland Communists.
This movie also probably holds the non-Woo squibs record.

In addition to the ones mentioned by others I might suggest Best Seller starring James Woods and Brian Dennehy. James Woods plays a pretty cold and ruthless hitman in that one and is very good in the role.

Au revoir, Shosanna.

MEMORY OF A KILLERa Dutch film about an older hitman who is about to lose his memory
BLAST OF SILENCEthis very dark 1961 film-noir indie is all about a New York hitman and how he lives his lifewell-made, it even looks like a European film in part due to its uniqueyou couldn't even tell it was low-budget, it was shot and made so well.

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While not movie, that HBO special called The Iceman, was pretty engaging. I think its available on youtube. In some ways he was an excellent hitman as he was able to balance being a cold blooded killer while maintaining a seemingly normal family life. Something Bishop could not do.

If you like comic books you might enjoy "The Killer" by Luc Jacamon. And for the record, "The Day of the Jackal" is in my opinion the best of the movies that follows around a contract killer (though a sizable amount of the film focuses on the police who are hunting him).

I would also add in the French film Le Samourai. Personality wise, very similar to the Mechanic.

Columbia Pictures Film Noir Classics
Volume 1 has a few you might like.
Murder by Contract
(1958) follows the life of a hitman with more than a few similarities to the Charles Bronson character in this film.
The Sniper
(1952) follows the life of a serial killer with the urge to pick off women from afar.
The Lineup
(1958) shows us a day in the life of a hit man and his psychopathic protege (Eli Wallach).
I agree with the recommendations of
Lon
and
Blast of Silence
.
Justin

i agree. that one is a great movie and a great reference. lol.
You've fallen in love with all the things in life that destroy men!

The Jackal (1997)
bourne series
pulp fiction

[Does the "shooting himself in the head" gesture with his hand]

Has anybody mentioned Leon? If not, I'd suggest that one.

LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON LEON

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Word, I thought I wrote this post.

umm, i knew that, i just thought that it was so good it should be mentioned twice. yep, thats it (phew, i think i got away with it)

Man Bites Dog
and
C'est arriv prs de chez vous
are one and the same movie, in fact.

then theres
ichi the killer
man bites dog
american psycho
mr brooks

A few recommendations from the top of my head (no particular order):
Le Samoura
Blast of Silence
The Day of the Jackal
(only the original version with Edward Fox!)
Ghost Dog
Lon
Henry: Portrait of a Serial Killer
C'est arriv prs de chez vous

That's an interesting view on the aquarium, tensaip. I thought that Bishop's issues at the aquarium had to do with his memories of falling in the lake as a child.
Forgive me for being picky, but when Bishop looks at the painting, he is studying the middle portion of Bosch's triptych "The Garden of Earthly Delights," not the Last Judgement portion. The middle portion depicts people enjoying all sorts of fruit and sex and sex with people who have heads of fruit and other weird stuff. It represents indulging pleasures and living in a state of sin. That fits nicely with your point that Bishop does care what Steve does after Bishop's death, and it suggests that Bishop feels a bit guilty about his trade, which is probably the cause of his psychological problems.

Steve being a true sociopath is his entire purpose for being in the story. That's not a plot hole, that's a plot point. Arthur is giving Steve the training Steve needs to kill ARTHUR! You can tell exactly when Arthur gets the inspiration to make Steve his associate when Steve tells the suicidal Louise: "if you don't care about your own life, why should I?" The entire first hour of the movie was devoted to demonstrating how Arthur's physical body is still a well-honed machine, but he's psychologically fragmenting. He has many authentic paintings in his home but the only one the audience sees him paying mind to is a copy of Hieronymus Bosch's THE LAST JUDGMENT. He collapses while visting an aquarium, when seeing the fish trapped behind glass triggers his own anxiety over being trapped in his way of life. And his "teaching" lectures are actually Existentialist musings to justify his continued living to himself.
Arthur found the dossier Steve was preparing about him BEFORE leaving for Italy, but took Steve along anyway. AND he booby-trapped Steve's car. If Arthur was living by the existentialist principals he was espousing to Steve, he would not have cared what Steve would be up to after Arthur's death. He snuffed Steve because he never trusted Steve not to misuse his training. In other words, Arthur had set up a trap that would have killed Steve regardless of the circumstances of Arthur's death.