Why do people say mean things when angry?

This is going out to guys and girls although my perspective is from dealing with men. My long term boyfriend and I rarely fight but we have gotten into a few heated arguments. When he gets really mad he says the meanest things and is quick to blame me for everything. I know anger clouds our minds but still. He also will make comments referring to breaking up and is completely unwilling to speak to me like an adult. In my opinion empty threats to end a relationship are off limits. Why can't people fight fair? Any and all comments welcome. No judgment here. I'd like feedback from people that may do this because I simply don't understand it.
Why do people say mean things when angry?
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