I don't kill villager(as my religious and kindness), but i need to kill iron golems for renewable iron ingots. But if i kill this(including villager) it still reducing my popularity to it. And what happen if my popularity is decrease? Also there are ways to kill iron golems without my popularity decrease?
A standard iron farm where the golems are auto killed by dropping them in lava isn't you killing the golems so wouldn't affect your popularity.
The same is true of villagers, if you push a villager off a cliff or into lava the villagers go into mourning and won't breed for the next 53 minutes but they're too stupid to blame you for the death.
If your popularity drops you can reset it by breaking all the doors, thereby destroying the village, wait a few minutes and replace the doors and you get a brand new village where your popularity starts over at 0.