I recall playing an MMO that gave XP based on who killed the creature as opposed to who did the damage. It used to be extremely infuriating trying to level up while some griefer followed you around stealing your kills.
In shooters though, its kind of annoying depending on the game. Similar to the MMO example, if you've had to put a lot of effort into killing someone only for that kill to be swiped at the last moment then yes, its pretty annoying.
If however the game you're playing makes it very easy to get kills and you've just been mucking around being a noob while someone else has come in and finished the kill then tough.
Context is everything.
In shooters though, its kind of annoying depending on the game. Similar to the MMO example, if you've had to put a lot of effort into killing someone only for that kill to be swiped at the last moment then yes, its pretty annoying.
If however the game you're playing makes it very easy to get kills and you've just been mucking around being a noob while someone else has come in and finished the kill then tough.
Context is everything.