I know nothing about the victim, but for the sake of argument assume he was a 10/10 scumbag and he did scumbag things to the defendant. The prosecutor will address the point you make, concede that the victim was horrible, but then explain the law that nobody is allowed to seek premeditated physical revenge for those horrible acts. The jury won't need to like the victim or endorse his lifestyle, in fact it is probably better for the DA that they hate the victim and find him to be a horrible human, but their disdain for the victim won't prevent a conviction in my opinion. I don't think that's how this works.

I'm also assuming that the internet version of the story will turn out to be +/- 50% from the facts when all is said and done, but taking rumors as fact this looks like an open/shut case. What I've read doesn't look good for the defendant at all. Video evidence of him being the last person to see the victim, if there was a fight there will be physical evidence, if there is a digital log of interaction on dating sites between the two, that would also be bad. If he went to the defendant's place with the intention of getting revenge, that's motive...also bad.