I know this is a wildly unpopular opinion, but I'm going to risk saying it anyway. I don't think Suo meant to laugh. I think Nirei caught him off guard and he was surprised. He smiles in shocking moments a lot, and you can tell by looking that the expression is startled, not something he puts on intentionally.
The idea that he laughed in Nirei's face to further drive a wedge between them didn't track for me, and I've noticed that people don't seem to agree so I've kept it to myself. I think the laugh had more to do with the mask slipping. His carefully constructed control of the situation was spiraling out of his grasp in real time. He's trying to keep an indifferent attitude on the surface while beating one of his closest friends bloody, and the guy still asks him if they're friends after.
I think he just lost it. That this is a sign of hysterics. Not cruelty or malice for the sake of the act.
I think this because of the evidence in that moment. If you're laughing in someone's face, you do it in their face. It's loud, it's intentional, it's insincere. I saw someone point out that he's so good at faking his emotions he even faked the blush.
He's trembling. The laugh starts as a pfft. It ends with him covering his face, still trembling. These are signs of someone trying to subdue a reaction, to contain it, not performatively put on a show.
He's having a giggle fit here. Now we know it truly ends like this, and I don't know about anyone else, but that panel just makes me feel a little better about my own theory on the whole thing.
At least Nirei's aware he's watching him come undone in real time.