if you use 2 commands one
/give @p 373 1 24796 {CustomPotionEffects:[1:{Id:21,Amplifier:200,Duration:999999999}]}
and the other one is
/give @p 373 1 24796 {CustomPotionEffects:[1:{Id:6,Amplifier:200,Duration:999999999}]}
you will get a potion of instant health and a potion of health boost. if u use the potion of health boost on someone their health bar will disappear an any damage at all will instantly kill them. if you then use the potion of instant health they will die... even in creative.
this works because any potion with a strength of 200 or higher will have the opposite effect of what it usually does. so it will even kill someone in creative and god mode. but sadly you have to be op for this command.
