They were not talking, both guys were shouting at each othe r, i'm pretty sure when someone is calling you out aggressively you expect things to go wrong very fast.When you're talking to someone, you don't expect to be punched. So, yes. Sucker punched. It's like us talking to each other now. Would you expect me to out of the blue, punch you in the mouth? No. You wouldn't.
Especially when the MC knows that the guy is the stalker and has been warned previously how unstable he is. There's no "out of the blue" situation building up there, you have an aggressive guy calling you out, you would expect what ? him shaking you hand ?