I'm very new to this, i'm not sure how I would be able to do it. It has to instakill anyone, no matter what effects they have on.
/execute at @e[type=snowball] run effect give @e[distance=..1] instant_damage 5 60 true
Put that in a repeating command block, and there ya go.
I'd use wither instead of instant damage, but this instant kills players and alive mobs.