It may be purely to spite Jordan, but the following weekend, Salem decides to do some investigating of his own.
He's fully aware that it's a bad idea, but his curiosity will evidently be the death of him. Maybe he just wants to prove something to himself, maybe he's self destructive, who knows? Either way, he doesn't think about it. He doesn't think it through at all, on Saturday night, when he makes the decision to watch Atlas' boxing match.
Again, he knows he shouldn't. Atlas kept this part of his life hidden for a reason. However, Salem knows Atlas isn't the monster everyone acts like he is. Salem has spent a lot of time with the guy, even if it's been a while. He likes to think that he knows Atlas pretty well. If Atlas was so bad, if he caused so much destruction, if he ruined so many lives, surely Salem would've known? He'd at least have suspected something. Salem can't even imagine Atlas committing a semi-serious crime like robbing a bank, let alone murder.