On land parcels with the damage option enabled, avatars can be damaged and killed (which simply teleports them to their home location, similar to llTeleportAgentHome).
The current health of an avatar is displayed in the menu bar (only when over damage-enabled land) with a red heart and the current health percentage (100% is the maximum, 0% means death).
Damage is inflicted when the avatar collides with a physical object that has a damage percentage set (using llSetDamage).
Avatars also take damage when they collide with objects (that do not have a damage percentage set) or land at high velocities, so it’s possible to use llPushObject to kill someone by pushing them into objects or the ground at high speed.
A damage object, colliding with another object that an avatar is sitting on, will inflict the damage percentage to the avatar.