From TF2 Wiki
Any death unaided by an opposing player is considered to be a suicide. A suicide can be caused by someone switching to another class or team while not inside either a respawn area or resupply area, a soldier's own rockets, and a demoman's own grenades and sticky bombs. In rare cases, an engineer can also be killed by his own sentry gun. If the player killed him self by switching teams or classes, the message, "<insert name here> bid farewell, cruel world!" will pop up in the recent kills menu (top right of the HUD) You can also die from falling damage in which case the words "<insert name here> fell to a clumsy, painful death." appear. Then, a skull appears.
In order to avoid suicide by these means, soldiers should be wary of their health when rocket jumping and firing at point blank range, and demomen should likewise avoid point blank encounters and be wary of the location of their own sticky bombs.
A suicide can also be performed instantly by the player by typing kill into the console. This is normally used in specific situations such as when the player is stuck and wishes to begin the respawn cycle.
Similarly, a player can enter explode into the console to undergo a "gib" suicide, as if killed by an explosion.
Unlike most online shooters, a suicide does not subtract a point from your score, although it does count as a death.
During Humiliation, you cannot commit suicide by typing kill or explode into the console.
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