There are two kinds of suicide in Myth. One is a tactic, the other is lame behavior.
The "Suicide" tactic is when a unit who has an explosive attack, fires at an approaching enemy at point blank range (or targets the ground past an enemy unit - ctrl-click) - which guaranteed to kill the unit but hoping to do some damage to the enemy unit(s) on it's way out. This is a respected last-ditch tactic.
The lame behavior "Suicide" is when a player does something intentionally to kill his units for no reason other than amusement or revenge or whatever other petty reason motivates players to try to ruin other player's fun. This suiciding often occurs in large team games where a player will blow up all his dwarfs or lock blast all his archers, hoping to make his team lose. This behavior is not respected by anyone.