Since you feel this spell is justified, it doesn't matter what anyone else thinks about doing it. It's your practice, you are allowed to do what you want. With that being said...
Revenge spells can work in a number of ways, using hair from the murdered cat, dirt from the death site, and a visual representation of the killer (a doll with a paper or toy gun for instance) would be a good place to start out.
When that cat was killed, there was probably an energy released into the surrounding area leaving behind a metaphysical imprint on the place. By collecting some of the dirt, you can then tap into that energy to send back at the killer.
Take a red, black, brown, or grey candle and lean the doll up against it. Have the doll point the gun at itself (This spell will Not kill the man, it's all symbolic).
Burn the cat hair and sprinkle the dirt over the candle and the doll, visualize that the bad things he caused will now cause him problems.
You can, if you'd like, say something along these lines:
"You've done wrong to my friend,
I'll put your crimes to end,
Feel my wrath and her pain,
Revenge is a two-way game.
You'll be sorry, trust me,
Forever cursed, you'll be."
Good luck with your endeavors, and please remember to be safe!