What happens when someone loses their humanity?

I've been looking this up for days now and I can't find it cause people say humanity is mankind. Well I'm writing a Harry Potter story you know just for fun and I need to know what happened to voldemort when he made the horcruxes. They say makeing one makes you lose some humanity so I guess that means he is less human I understand that but I want to know more about when someone becomes less human cause I think voldemort seems human just looks different and seems eviler but was he really that good before he made the horcruxes I don't think so. So please if someone could please send me a website that explains it or explain it to me please do so.