A grudge aka revenge kill afaik is when after respawning, you kill the enemy player who killed you last time. The condition for it being a grudge kill is that he should be the first player you kill just after you respawn.
"Warriors of fate, you shall dance bravely. Use your golden wings, and fly to the skies. "
CptDesu posted... Revenge Kill: Kill the person who killed youGrudge Kill: Kill the same enemy 5 times.
For a revenge kill, I believe you need to be the first person to kill them after you respawn from them killing you. If someone else kills them before you get to them, it's not a revenge kill. I'm pretty sure I read that in the in-game text.