Insta kill mob if instant damage or health set to 30 or more.

Since instant damage on living mobs and instant health on unded does not damage them, but trigers the hurt animation is a very powerfull tool for mapmaking. But it has some wird effects.

If you do:

summon zombie
effect give @e[type=zombie] instant_health 1000 30

and then you punch them, they instantlly die.