In almost all multiplayer first-person shooters, death is temporary. After a brief moment, the player respawns somewhere else in the level, ready to fight and kill (and die) again. Such is the nature of deathmatch. Players generally don't spawn in a completely random location but in any randomly selected point preset by the map's creator. This is necessary so players don't spawn stuck in walls or other immobile obstacles. This creates a situation that experienced players can exploit.
Knowing the spawning points throughout a map can enable a player to make what is called a spawn kill: A player can take up a position giving him or her the ability to pick off players almost immediately after they respawn. If there aren't enough spawn spots (or they aren't spread out enough), a spawn killer (or several spawn killers) can effectively neutralise his/her opponents by continuously slaying them before they have a chance to do anything. Spawn killing is generally frowned upon... with a vengeance.
There are several ways to reduce/eliminate spawn killing within games. Some games have a setting that allows a certain amount of time after a player spawns during which that player is invunerable (sometimes at the cost of not being able to attack anyone until they can be harmed). An example of this can be found in Fox Interactive's Aliens vs. Predator.
In games in which there is no option to create a brief moment of invunerability, the game server's administrators can merely state that spawn killing isn't allowed. Spawn killing while an admin is around would most likely then result in the spawn killer being kicked from the server (or at least warned). Many games provide the ability for a player to be kicked out of or banned from a multiplayer game if enough of the other players in the game vote to do so. This more democratic method can, of course, be abused but such abuse is (in my experience) quite rare due to a combination of apathy and eye-witness testimony (if someone's been spawn killing left and right for a while, chances are it's been observed, if not experienced, by plenty). What does happen sometimes with such votes, unfortunately, is in a game with teams, an entire team will vote in support of a spawn killer provided said spawn killer is on that team.
There are times when spawn killing is somewhat unavoidable for the player's character to stay alive. Running through an enemy base in a capture the flag game, for example, might present a situation where the intruder ends up face-to-face with a newly (re)spawned player. In such a situation, the choice is usually to spawn kill or to get killed. There really isn't much that's rude about spawn killing in this situation. It's laying down head shots from across the map before anyone can do anything that's generally looked at as the bad thing.