Technically, if you allowed a save to avoid the effect entirely (probably a Will save) then added a 50/50 chance of death it would be much weaker. Basically finger of death with a 50% miss chance and the ability to dispel it before it activates. So that might be as low as level 5.

Although, just to get nitpicky, shrodinger's box doesn't have a 50% chance to kill the cat, the cat is simply neither dead nor alive until the box is opened...or it is both. The chance exists for the cat to still be alive, but it is extraordinarily smalle (assuming a good poison was used)