A couple of them died during an attack outside the gates...
The guards outside the Diamond city are fixed and respawns. They can be easy killed by hostiles, but if you try, you may fail. Youre not suppose to kills guards unless you want to play evil. The hostiles can easy be killed by you, but the guards may have problems. It depends on you lvl of course.
Cheers,
-Klevs
Yes they respawn.
If you're thinking of the same part as me there are 2 or 3 guards fighting a small group of super mutants in a ruined 2 story building, the mutants are firing down on them from the scaffolding. This event happens every few days.