Why would he tell me to kill myself?

Why would my now ex boyfriend tell me to kill myself if I asked him a simple question about if he was messing with his step sister? This is a long long story that I don't really want to explain again but.. there has been times where I accused him of possible sleeping with other woman.. today I told him if anything has happened between him and his step sister again and after a few minutes into the conversation he lashed out at me saying I should kill myself and cut my
throat open again? The first time I did that I was not with him at the time I was with another person and confided with him about that.. if he didn't do anything and he wasn't guilty about anything why would he have to tell me such thing about myself? Especially when I was sitting right there with our baby? His step sister sent him a picture of her in some penguin outfit that he didn't replied back to but was persisent enough to text back again asking about our son.. which he then replied back to about but our son wasn't even with him at the time and he lied saying he was going to give the baby a kiss goodnight for her. She also bought HIS dads house recently because the dad died and he gets checks that are sent over there which he claims he hasn't picked up yet because of me and what I think and that he feels bad he doesn't talk to them.. what do you think?
Why would he tell me to kill myself?
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