A person who regenerated after being spawn killed.
Spawn killing is, in video gaming terms, an act in which a player kills another player on the spawn (or spawn area) usually as soon as he/she spawns or as any form of temporary protection from damage and/or death, such as forcefields, dissipates. It is usually done for a variety of reasons: to get revenge on another player for doing something he/she didn't like, such as spawn killing itself, to prevent them from attempting to get involved in a war that is occurring at a place (i.e. Solar Conquest by Cocoawarrior), or to simply just to get knockouts. It can tend to be very aggravating for the victim(s) of spawnkilling at a place to be caught being spawn killed by enemy player(s), which causes the victim(s) to feel enraged, possibly to the point that the player would use a very powerful/overpowered (also known as OP) weapon (if any is available to said player) and attempt to slay the player's nemesis as a revenge, or that the player would "rage quit", which is a gaming term for quitting a game out of anger for various reasons. Spawn killing is also known by many players to constitute griefing, in which a player deliberately irritates and/or harass other players who are playing in a game.
Often times, newer users will false report players because of this.
Force field killing
Force fields do help on countering spawn killing, but only to an extent, due to the fact that if the force fields are active on the player and the player fights another enemy player with their own weapon, the player would perform an act similar to spawn killing called "force field killing". Force field killing is equally frowned upon as regular spawnkilling by many players, as the force field may prevent the player from taking most damage from many sources, which allows the player with the force field to safely fight any incoming enemies, including spawnkilling enemies, more easily.
Some weapons, such as the Frost Blade, can pierce through force fields.
Prevention
Usually, spawnkilling and force field killing come from the fact that a place was not well designed, due to the poor design of the map and/or the weapons that are present in the place. When weapons are designed correctly, it is easy to prevent both. For example, a simple fix to both would be to give a force field to players when spawning, make the weapons so that they use the TakeDamage method instead of directly changing the health (which is bad practice for weapons anyway) and make the force field disappear when a weapon is selected. There are also many other ways to prevent it, such as only giving the weapons to the player once the force field has disappeared.
Unfortunately, as most place creators do not have the knowledge required to implement a simple fix like the ones mentioned above, and instead just insert the weapons directly from the free models, spawnkilling and force field killing are both problems that frequently happen.