Bodies not put to rest, especially those who have died on nights with no moons, may not release their souls properly and will eventually become corrupted. These souls may start to move their bodies again, creating walking corpses with darkening motives.
Zombies constantly ooze their slowly-dying souls from their wounds and rotted openings.
In early-stage zombies, there is a chance of reasoning with them. Their souls may not be fully corrupted and they might be able to understand you. Zombies will often show signs of the interests they had in life; collecting flowers or attempting to paint are some of the more depressing instances that have been seen.