I don't know if this one rises to the level of an "overused" plot device just yet...but I've seen this twice in the past 2 weeks so if it crops up again soon it might get there:
-A superhero has a "no kill" rule, sticks to it for multiple seasons, etc. They hold back when fighting the bad guys and beat them so badly that any real wold humans would die, but the bad guys always survive.
-But then after the audience sees this happen for years, one day the hero encounters a bad guy so evil that they kill and/or dismember a secondary loved one that we've seen from day 1.
- The hero catches up with the evil bad guy and they put up a surprisingly strong fight, and for some reason they are able to keep up with the hero much better than expected.
-Thinking of their dead/dismembered loved one, the hero fights in a fight of rage and (shock!) they actually "Kill" the bad guy! OMG they broke their no kill rule!
-But then...at some point the camera focuses on the mutilated "corpse" of the dead bad guy...and

they take a breath!!!
The hero broke their "no kill" rule...and the bad guy didn't even stay dead!!!