Harry didn't know about the Goblin trying to kill MJ. He knew about that whole parade thing, but so did the rest of New York. Not to mention the fact that Harry was already mad at Peter and MJ for them having a thing while MJ and him were still a couple.
Add that to the fact that he's obvioiusly gone insane and well...it's almost obvious.
I think it's too early to confirm if he'll be evil or good...you can't judge it by the films...in one hand it looked like he began to understand why peter killed the goblin, on the other hand he was indeed going nuts but why? Because he realised that his father was the evil Goblin...or was he getting nuts in general pure out of revenge?...I'm just thinking about what I saw in the comics altho they do not always follow the comics of course...