Undertale spoilers
@kara_dreamer@witches.town Without his Soul he can't feel any remorse or empathy or love or anything, it's all hollow to him, which drives him further towards hate and hating both himself and existence and everything, the only enjoyment he finds is in finding new things to destroy and hurt and manipulate. Even when he captures the six human souls it's not enough to grant him a true soul. It takes EVERYONE'S Soul to do it, and even then he's too tortured by his past to recant.