[1] Kill stealing is common when the rewards for finishing enemies off are highly desired within the game.
Players usually ignore complaints of kill stealing in FPSs because the rewards are less significant and because these games move much faster (i.e., it was probably accidental).
(In situations where they are, such as two opposite-faction players in a World of Warcraft player-versus-player server, there is usually little animosity towards kill-stealing, as there is a means of redress and prevention, and it is seen as part of the general struggle between Horde and Alliance) In team based game modes, many players overreact when they do not understand the objective is for the team to win, rather than individual progression.
Many newer MMORPGs implement game designs that distributes the reward more fairly to those who fought a creature.
The player then gets awarded the amount of damage they did to the enemy as points, and on their statistics it counts as kill rather than as an assist.