Agreed.
I mean, it's obvious he allowed Mordred to mortally wound him (out of guilt maybe or misplaced sense of responsibility to die alongside his own son). It's not like he's surprised/terrified/afraid or anything but resigned after killing him.
His time has passed, and like Merlin before him, he now has to retreat and fade away.

I believe he did.
To be rid of all the "evil" that was done when he fathered a son with his sister. Remember, God struck Arthur down with lightning when the abomination was born into the world.
And: no weapon forged by man could destroy Mordred while he had on his magical armor. It took Excalibur to get to his heart.
That was an excellent scene.

I think you are not alone in thinking Arthur used Excalibur: the sword is so famous, it seems only logical that the king should use it to slay his enemy. Thus, I think that even if Malory's version (which is most English-speakers' medieval reference) has Arthur gutting Mordred with a spear, and even if it is that scene which Arthur Rackham has so famously illustrated, that illustration (and other famous ones) got misinterpreted and long-lastingly embedded the image of Mordred's spear and Arthur's sword in the final battle.
http://www.timelessmyths.com/arthurian/gallery/mordredarthur.jpg
"
Occasionally
I'm callous and strange."

Because of this film, I was always under the impression that Arthur used Excalibur and Mordred used a spear during their battle, but some texts suggest that Arthur uses a spear and Mordred uses a sword and it is Arthur who stabs Mordred first before Mordred rushes through the spear and mortally wounds Arthur.
My question is, does anyone know which version is more prevalent?

Arthur was also exhausted after the battle. I think he knew the only way he could get close enough to Mordred to kill him was to allow Mordred to run him through.

I took it to mean that death (whether by Mordred or any of his men) was Arthur's destiny, as he himself explains to Guenevere at the Nunnery?
Casting Excalibur away was the trigger for Arthur to be carried off to the Isle of Avalon.

I always had the suspicion that Arthur wasn't really expecting Mordred to be capable of what he did, and so the spear thrust caught him by surprise. It was his son after all. Mordred was a disrespectful little brat.

Are you writing a paper or something?