to spare: to hold back from harming, destroying, or taking something that lies within one's power, often out of pity
This word lives at the meeting point of two ideas that English usually keeps apart: the inward softening of pity and the outward act of holding back. To spare in this sense is not merely to feel sorry for someone; it is to decide not to strike, not to seize, not to finish off the one who is at your mercy. That is why the same verb fits a princess melting at the cry of an abandoned baby and a conquering army choosing whether to leave anyone alive.
Because the act of sparing is the point, the word turns up most often in the negative, in the warnings and judgment speeches of the prophets: God, or an invader acting as God's instrument, declares "I will not spare," meaning no one will be held back from the sword. The horror of those passages depends on the tenderness the word normally carries. To say God will not spare is to say the restraint that pity would ordinarily produce has been withdrawn.
The two shades below, holding back from destruction and taking pity on the helpless, are not separate words but two angles on one motion. Sometimes the text foregrounds the protective action (a king spared, a city not spared), sometimes the felt compassion that prompts it (a daughter's pity, an eye that does not weep for an outcast). Many verses carry both at once, which is exactly why one Hebrew word covers the whole range.