Boyfriend and ex girlfriend

Would you be mad if your boyfriend liked his ex-girlfriends pictures on Instagram? I know that it sounds stupid but it bothers the crap outta me. He says he's just doing it cause it pisses her off cause she hates him, but honestly I don't think she hates him at all. She still tries to talk to him and he doesn't message back but he still likes her stuff on social media. I don't know I guess I'm probably overreacting, he's never cheated before and I have his social media passwords so he isn't hiding anything from me that I know of. It just makes me feel insecure even though he assures me that he hates her..