What would you do if you heard your parent threaten your life?

I’d probably laugh, it’s a figure of speech, not an actual murder threat.

Yeah, @KNOWITALL has to be right.
Surely this was said in jest, millions of parents have said something close for generations.

Only you heard your aunt & so hopefully this theory will ring true.

I do what @KNOWITALL said.

My ex has said the same thing about my son and my daughter at times. I have told her, “it’s not fucking funny.”

My ex’s father would get drunk and shoot a pistol off in the house when she was growing up.

I don’t see it as amusing, but most of the time it is an exaggerated comment of frustration. It simply means they want someone to understand that they don’t condone a particular behavior, and that they have probably taken every humane measure they can think of.
Sometimes someone under influence of drug, alcohol, or mental defect will actually mean it as a threat, but mostly not.

I agree it is a cringey thing to hear.

There’s nothing funny about it, but it’s sadly a figure of speech for some.

If my father had threatened me with death, I would have welcomed the threat much more than the broken nose he gave me in the fight we had. (He sucker punched me splattering blood all over the four walls of the bathroom where the fight took place. Talk about “close quarters combat!”) If I had any love for my father, I lost it that night. And if there was but one bullet for my .22 rifle, I would have shot him.
The threat of death was, in your instance, just figurative in nature. I might really want to kill someone but saying it and pulling the trigger are two very different things entirely.