FireLance
Legend
I think a better approach may be to generalize the rule. As I posted in another thread:4e got around this with the minion rule - they never took damage on a miss. We don't know that 5e won't have a similar rule.
Maybe we should have a rule that a missed attack cannot reduce the target's hp to 0 or below. That way, the final blow that kills an opponent will always be a hit.
If you like, you can also flip it around for saving throws, i.e. if you pass your saving throw, you will always have at least 1 hp left.
For spells that deal automatic damage, such as magic missile, the wizard must make an attack roll to see if he "hits" and kills an opponent with low hit points. Alternatively, the low-hp opponent must succeed on a Dexterity save or die.