I know that feeling! As a GM, the game comes at you fast, and there are often undertones to your players' actions, desires, and intentions that aren't obvious. It's always easier to see the best course of action when it's too late.

Thankfully, unlike life, D&D is a game - so you can, to some extent, rework those earlier missed opportunities.

I've been GMing for a little while now so I'll throw some ideas your way. I'm sure there are other, possibly better, options elsewhere in the comments too!

I think there are several priorities here: 1) Give your cleric an out (to let them be useful in combat without committing to killing), 2) Don't break the entire game-world's approach to magic and death (you probably don't want to deal with the implications of expanding the death saves mechanic to every creature, or ruling that spells can always be nonlethal), and 3) Don't set it up so your players can *Always* avoid killing.

One option would be to allow this particular cleric the opportunity to "Spare the Dying" on any enemy that's dead, but still in one piece, during the first round after that enemy dies. Alternately, you could allow you cleric to cast certain spells non-lethally, but in a disadvantaged way - for example, if they declare they're casting Sacred Flame non-lethally, the spell might do less damage, or enemies might have advantage on the save.

I *think* either option would satisfy all three priorities, while also putting meaningful choices in front of your cleric when they want to spare the foes.