Forgiving an unforgivable?

Something I've never really understood is unforgivables. All spells from what I understood come from mind bending want. I want something shoved? Spell. I want something to float? I'll make it so. I want to protect someone from Voldemort though? How come Lily for example upon hearing someone murdering her husband downstairs didn't arm herself by the door and blasted something to just kill him?

At what point is cruelty a necessity to survive? If a person has bad intentions, what if they save people? If someone had casted the killing curse at Voldemort, would they be a hero or someone just as bad? Would they suffer the same way as someone who doesn't have good intentions? Spells rely on intentions, so if I cast the killing curse at a bad person, saving millions, would I be effected even if my wants were good?

edit: everyone has been very insightful, thank you