Made a joke about killing me.

The other night my boyfriend was saying how our daughter only has his last name, but the thing is she actually doesn't at all legally speaking, she only has my last name. He was being rude about it and I said we can hyphenate it but right now she literally only has my last name. And then he said "well I guess I'll have to kill you then so she can take my last name." He didn't laugh, he didn't say just kidding. It was not in a joking tone.