Wiktionary
kill stealing
vb. (present participle of kill steal English)
Wikipedia
Kill stealing
In multiplayer video games, particularly in first-person shooters, MMORPGs and MUDs, kill stealing is the practice of obtaining credit for killing an enemy, when another player has put more effort into the kill. This usually happens when a game only keeps track of which player defeats an enemy. If one player whittles down some enemy's health points, but a different player eventually finishes the enemy off, this second player might obtain all of the loot or experience points from the enemy. Kill stealing is common when the rewards for finishing enemies off is highly desired within the game.
Many players feel that kill stealing is a dishonorable practice.