Because that was supposed to be "the end" between them, he was never going to chase him anymore. And Minjae preferred kissing him rather than hitting him. I think it's an understandable choice, given the supposedly situation.
Minjae actually slaps Hyeongjo in the end, but not now. But I like him more when he chose to kiss. Uhm, I think "kissing" is more suitable to his character - intraversion, graceful, tolerated but strong and straightful in one or another time. Maybe he don't want to use vionlence to Hyeongjo just like the way he had to bear. Remember what he said: "I didn't deserve the way you treated me". The kiss makes me like him more.
I don't get why didn't he hit him? :C
I mean yeah, you probably love him at this point right, but dude... He raped you several times and at least you could have released stress a little.
If you had slaped him, he could still continue with that forced kiss.
I like that manga ok, i just think a slap would have been better. (￣∇￣")