A just god wouldn't allow evil to exist. Evil exists. Therefore god does not.
Once again. The problem of evil is a real thing, and I am sorta offended that you think it is stupid.
The argument is flawed in lots of ways. For example:
1) It blindly assumes a just god. Why? Like kaijyuu says, what if god happens to be a dick?
2) It ignores the fact that if god exists..."justice" would pretty much be defined by that god. By definition. It doesn't make sense to assert that god exists...but then use a subjective, personal measure of justice. If god exists, then he's the one making the rules, right? So...what if "the rule" happens to be that "believing in He, Him" is more important than insignificant stuff like killing you petty humans? Using the bible as an example, if you look up the ten commandments, I think you'll find that
several of them can basically be summarized as "gimme respect, or else" and "don't bow to any god other than me or I'll punish you, your kids, your grandkids and their kids" is listed a couple of steps above "oh yeah, don't kill people." Again, if there's a god, he's obviously the one making the rules, and it doesn't make sense to apply your own rules to god just because you happen to like them.
3) There are perspectives that reconcile the issue of evil. Obvious example: imagine that god exists, and that "life" is something that souls do voluntarily and temporarily, with temporary memory loss as part of the deal. When you die, you remember all your previous lives, and whether or not you do it again is totally up to you. "Life" becomes very much like a game or a stageplay. Saying that god "wouldn't allow evil" given such an arrangement is like saying that god wouldn't allow Hamlet to suffer, or players to be fragged while playing Halo. Sure, from the perspective of Hamlet, things are probably pretty horrible. But the actor playign Hamlet chose to get on that stage, and god isn't going to pull him off and tell him he's not allowed to play Hamlet, and he's not going to "magic" everything on the stage back to happy and nice. Why should you expect god to "stop evil" in your life?