Where the motive for the killing includes an appropriate sense of vengeance.
Deaths that will definitely fall into this category will be those where the killing was not necessary, but the desire for retribution for past offences means it takes place anyway.

Killings that have additional motives may also qualify, provided that the victim has caused sufficient personal distress to the killer. This will have to be above and beyond just trying to kill them too. It would have to be the kind of deed that leaves a burning, festering desire for bloody satisfaction. We're talking slicing off their testicles, killing their mother, or deleting their lifetime collection of downloaded pr0n types of offences.