Usually snipers 1 hit kill me with their machete (crit) when I am a demoman at 80% health.
this is because good snipers (as in, the ones that would notice you and thus attack you all with the machete) are terrifying damage-dealing monstrosities and had a high chance to crit you by default (since he was using a melee weapon) stacked on top of the additional bonus he got from doing all the damage he did previously to people who werent you
they probably have like a one in four chance to murder your stuff in, either fight the sniper from mid range or forgetin bail