*SPOILER DISCUSSION* You'd have to be stupid to let him go....
Okay, title said spoilers. Title is deliberately vague. Don't continue reading if you don't want the final act of the movie spoiled.
Okay, so your brother-in-law has been violent towards your sister. You are seriously angry with him. So you tie him up in a warehouse, clearly meaning business. You also know that this guy is a bit unhinged and threatened you with a gun, pointing it directly at your face.
So with all that in mind, I recognise that injecting someone with HIV is pretty horrible (though I'd also note that exposure doesn't actually guarantee infection), but if he was unhinged before imagine how he'll be when you've done that? Seriously is your sister going to switch to a new apartment in the meanwhile? Are you all going to skip town? Heck, he's seen the faces of everyone involved, so couldn't he quite easily take legal action (after all, he's got nothing left to lose, has he)?
Louis should have known that Leo was going to gun him down. There's no reason why this should have been surprising to him. He knew Leo was unhinged, Leo had threatened him with the gun before and, after what Louis did to him, Leo had a massive incentive to do something drastic.
This is pretty much my only problem with the movie. I'd quite like it if they'd taken the racism angle a bit further since that just seemed to be dropped part way through the movie. But by far my biggest issue is this strange decision to just let Leo go.