Why do guys become so mean to the girl when they break up?

I’m not asking for any personal reason, I’m genuinely just curious from a guy’s perspective.

I’m talking about (hypothetically) if the guy breaks up with her, and he becomes mean and rude towards her but still watches her social media posts and gets angry when she talks to a guy/guy friend.
Why do guys become so mean to the girl when they break up?
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