Spawn killing is an act on Roblox (and many other games) in which a player repeatedly kills another player on the SpawnLocation or spawning area, usually as soon as they respawn or as any form of temporary protection from damage and/or death, such as ForceFields, are removed from said player. It is usually done for a variety of reasons: to get revenge on another player for doing something they did not like, such as targetting or taunting or to farm knockouts. It can tend to be very aggravating for the victim(s) of spawn-killing at a place to be caught being spawn killed by the 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 an act of 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.
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 spawn-killing by many players, as the force field may prevent the player from taking the most damage from many sources, which allows the player with the force field to safely fight any incoming enemies, including spawn-killing enemies, more easily.
Any weapons that disassemble limbs are capable of killing players with forcefields.
Usually, spawn-killing and force field killing comes 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, like most places, 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, spawn-killing, and force field killing are both problems that frequently happen.