From The TimeSplitters Wiki

A Kill is when one player causes another player to die by depleting their health with weapons. Players sometimes show of their gaming skills by bragging about their amount of kills or kill rate.

Points and Opposites

In most game modes a kill counts as a point, and a certain number of kills/points are required to win. The opposite of a kill is a Loss, which means the player was killed by another player. A loss usually makes the player lose a point, unless the scoring system is set to "Kills" only.

Effects

When one kills a player, that player suffers the effects of a loss. If you kill a Virus that virus spawns somewhere else, however for a short time their body still has the effects of being a virus. The same thing goes for Flame Tag. If you kill someone in an Elimination match, you make them loose a life. In Vampire, kills are required to stay alive, as they replenish your "blood".

One Shot Kill

A One Shot Kill is when a single projectile is 100% lethal. The best weapons for these are Explosives, although if you put the setting One Shot Kill to on, any weapon is a one shot kill weapon.