Damage, however, is not the only concern. One thing that's clearly taken into account is AOE. You mention Circle of Death and its low damage (same damage as 3rd level fireball but using a 6th level slot) – but Circle of Death has an absolutely massive area, 9 times that of a Fireball.The 2014 Dungeon Masters Guide (page 284) advises how much damage a spell should do per slot. However, similar to how the Monster Manual ignores the DMG advice for monsters, the Players Handbook ignores the DMG advice for spells. For example, the DMG says a 3rd-slot spell should deal 21 damage (6d6), but no spell like this exists in 5e. 5e spells deal amounts of damage that are wildly inconsistent. Even so, it is possible to identify certain spells as bench marks that other spells can measure against. Whence: multi-target should deal about 7 damage per slot and single-target should deal about 9 damage per slot.
Power Word Kill deals no damage. It kills a target with 100 hp or fewer, but if the target has e.g. 101 hp it has no effect.Instead of the scaling single-target 81 damage, Power Word Kill deals 100 damage.
It is essentially the same as Sleep, only a higher-level version.