I'll be honnest: I'm a big shipping trash, and I'm squealing a bit seeing this, but I don't think Mashim is going all berserk on the creepy guy because Ame sacrificing herself for him made him realise he loves her. Like Mashim said, he's not interested in romance, but we know he have a sense of honor and he can feel respect, mercy and friendship for others. I think it's more a "So, this girl was brave enough to shield me from this attack? Okay, I appreciate that, and I will avenge her!" kind of situation.
Still, maybe I'm wrong and the ship is sailing! o/