I think of “suicide mission” as someone intending not to return alive. Something like the 9/11 hijackers, or suicide bombers, or Kamikaze pilots.
In other words, 100% fatal on purpose. Someone who runs into a burning building is not on a suicide mission, no matter how dangerous it is.
Certainty of death
Small chance of success
What are we waiting for?
Gimli the Dwarf