A Sacrificial KO, as the name implies, is a knockout in the Super Smash Bros. series wherein an attack or maneuver executed by an aggressor leads to both the knockout of an opponent and the attacker as well. Sacrificial KO's are useful generally when the "kamikaze" has a stock or score advantage or when a match can be won outright by a sacrificial attack, or to weaken a powerful player by removing their stocks.
It is interesting to note that while some Sacrificial KO's are performed even when stock or score are even, they do present an opportunity to "even the score". Some players regard the sacrifice as worth performing even when the stock or score are even in order to reset the damage percentage if the character who performs the technique is at a percentage disadvantage. Other reasons may include vengeance or spite.
Notable examples from Super Smash Bros. Melee include Donkey Kong's ability to leap from a platform while grab-carrying, Fox's Firefox (aimed with no hope of recovery), and various meteor smashes executed too far from the edge of a stage. The down special move executed by Captain Falcon, the Falcon Kick, can also be used in this manner.
If a player uses "Bowsercide" (using Flying Slam) on an opponent, Bowser will die first, even though both players died at the same time. It is not recommended to use when both players have one stock, as the victim will win at the end of the match. If a player uses "Ganoncide" (using Flame Choke) on an opponent, the victim will die first, resulting in the Ganondorf player winning if both had one stock left. Oddly enough, if the victim did not take any damage but was caught in a "Ganoncide", the match results will say that they self-destructed, even though they were caught by an attack. The most known Suicide kill is Kirby's Swallow (Swallowcide). To do this trick, the player must fall off next to a cliff and swallow the enemy whos near it. But, the opponent can escape quickly to avoid this trick.
In Super Smash Bros. for 3DS and Wii U, new characters have access to unsafe Sacrificial KO's, such as Robin's "Robincide". In this KO, Robin catches an opponent in Nosferatu offstage, Robin and the foe will slowly float to the blast-line. This move is unsafe because after it finishes, Robin becomes helpless, while the other fighter still has their jump/recovery. Because of this, some characters may be able to save themselves with good timing, while Robin will definitely lose stock. As competitive play generally has a "no holds barred" attitude against any manner of combos or techniques, a Sacrificial KO could be considered as a legitimate part of the arsenal of a player who holds to this consensus to win at all costs, no matter what others may think.