From Halopedia, the Halo wiki
For suicidal enemies or canon instances of suicide attacks, see Suicide attack.
Suicide is the gameplay term given to when a player kills him/herself.
Whether intentional or by accident, the player may commit suicide through various methods. While this is not the objective of the game, and does not benefit the player in any way, it is a regular occurrence during a game session. Reasons for committing suicide may vary such as sheer boredom or mere curiosity.
One of the common methods of committing suicide in a game is by using their own weapons on/against themselves; an example is standing near a self thrown grenade, or firing any rocket launcher-type weapon at the player's feet or nearby wall. Other methods could be by using vehicles, falling from great heights, or simply by venturing into dangerous places (i.e. cliffs, kill barriers).
In campaign and multiplayer
Committing suicide can be a strategic maneuver in a campaign and multiplayer context (typically in a team-based gametype). In a cooperative play in a campaign session, the players can task one another to be a decoy to allow the other to consider options with dealing with the situation at hand. For example, in a Legendary playthrough, Player One runs into the enemy to distract their attention to allow Player Two to engage said enemy. Also, an uncommon tactic that could be added to this would be to stick a plasma grenade to Player One as he/she runs into the enemy.
Similar tactics can be employed in multiplayer whereby Player One of Team A wielding a rocket launcher rushes into the opposing team's base, killing himself/herself and the opposing team to allow Player Two of Team A to retrieve the objective. However, it should be noted that committing suicide in a multiplayer will usually impose a respawn penalty on the suicider. In Slayer-based gametype, a suicide will usually deduct a point for the suicider and, by extension, the suicider's team. In Infection and Flood-based gametypes, a suicide will also usually result in the suicider immediately becoming an infected player.