I think Billy killed tatum because stu was at the door seeing everyone out. Billy appeared outta nowhere at the front door right after tatum was killed.

Excellent point. Definitely would make more sense with how you perceived it. And also the fact that Stu actually explained how you would gut someone. Knowing that he is one of the killers, it would seem more plausible that he would have known how to gut someone if he did it.

I took it as him just silently telling Stu to stop causing suspicion As if to say "Stop being defensive like you've got something to hide when no one is seriously accusing you of anything."

I think it was likely Stu who killed Casey and Steve with Billy doing the calling, but what has always caught me off guard is the next day, while sitting by the water fountain, Stu says that he didn't kill anyone, and Billy replies saying, "Nobody said you did." I always took that as a possible hint that it was Billy who killed them.

First off, there are a million of these threads on here. Why make another one when you could read those and figure out what others said? You could even reply! Hmm, who knew?!
Second, when it comes to Casey and Steve, Stu killed them and Billy did the phone calls. Kevin Williamson himself has said this over and over.

Iv always thought it was obvious Stu did the most of the killing and Billy was more than happy to let him. Billy was in this to get revenge me on Sidney and her mother Stu was in it for the thrills and bragged about that.

I do think it's possibly Billy let Stu do all the killing, if only to keep him satisfied. Still, I think Billy killed Tatum. Stu was hosting the party, Billy doing the killing gave him an alibi and control over the situation inside the house.

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I believe it was definitely Billy who killed Tatum, cause of one previous scene where Tatum said to Sidney, "Billy and his penis doesn't deserve you." So Billy had enough reason to kill her, and plus, he needed her out of the way for the plan to work.
As for Himbry, I also believe it was Billy, cause after his school conversation with Sidney (before the bathroom scene), he just becomes absent from the film until the video store scene. What exactly was he doing during those 20 minutes or so out of the film? And maybe Stu may had chipped in to knock on one of the doors.
Hopefully these two tidbits were helpful here.

Why even bother doing that? Billy could have just left the costume in the garage where he killed Tatum. Stu sent her there to die anyway, and the last we see of him, he's heading to the garage for another beer, where her corpse is.

When it came to Casey and Steve, Billy called while Stu did the killing. As for Tatum, Stu was the one who sent Tatum out to the garage in the first place and stayed inside while the party went on. Billy was the one who killed her. As for going back in the house after Tatum was killed, you'll note by the layout of Stu's house that you had to pass through the laundry room to get to the garage from the kitchen. Billy went into the laundry room to remove his Ghostface costume after killing Tatum, doubled back through the garage, and made his way to the front door to greet Stu and Sidney.

Well, Billy had a motive seeing as how it was clearly stated in the movie. Doesn't mean he wouldn't kill anyone that he didn't have a grudge against, but still. Funny how you keep repeating that serial killers don't need a motive to kill (something I've never claimed otherwise), yet you ascribe most of the violence in the movie to the guy who had a reason to be set off.
And it's far more likely that Stu was the one who killed Casey and stabbed Dewey. Stu kills only two people? Nah. Especially seeing as how Kenny's death was completely spontaneous.

What I said:
No motive
. They killed because they enjoyed it. No motive required.
They killed Sidney's mother together.
Billy killed Casey, Himbry, and Tatum. And he stabbed Dewey.
Stu killed Steve and Kenny. And he was mostly the one who chased Sidney around, save the police car scene, post Dewey stabbing.
That's my view, anyway.

That's my whole point Stu didn't have a motive, Billy did. It's a point of discussion to wonder if Billy was only really interested in getting revenge on Sidney and her family and let Stu kill the rest.

People keep going on about "motive", but serial killers, real or fictional, don't necessarily need a motive. To coin a phrase: "The kill is the thrill". They both liked killing, and they both killed. Simple as that.