Responses
Perhaps because he's actually good to the ML ONLY. He's clearly an ashole to everyone else, but with the ML he plays the sweet card. The ML is kinda dense at times too with the things he says, & he didn't see how hard Ai was trying to be his friend. So it's possible he doesn't see the true character of the evil spawn, specially because he considers him a "friend".
It could also be guilt because he knows the evil spawn likes him, but obviously the ML had 0 interest in him. Plus they've been friends since they were kids.
The ml keeps saying that haraguchi is a good guy but I think he's just delusional. All I saw was an absolute failure of a human being till now. That was the case when he was a child and even more so when he's grown up. I wonder what the author's idea of a normal person is if haraguchi is considered good.