Murder Suicide is the act of killing an individual and then committing suicide directly after. The causes for Murder Suicide cases are usually attributed to one of two factors: it was premeditated (the killer intended to murder their victim and then kill themselves) or it resulted after the murder (due to emotional guilt and an intense feeling of remorse). Most murder suicide cases occur between lovers and family members, as it is rare for a Serial Killer or Murderer to kill their victims then "off themselves". A murder suicide MAY be considered in the event that this does occur however, where they intended one final victim before their death and they ended their own life directly after.
Separately, the event of "killing oneself to stop another" is a different concept all together. If an individual commits Suicide by approaching the people they intend to kill while putting themselves in considerable danger (either from the intended victims themselves or the method of killing them such as an explosive), this is not Murder Suicide. Rather it's often called "Kamikaze", based on the Japanese fighter pilots in World War II that would crash their planes into the opposing force either as a last ditch effort or as their original plan of assault.