Throughout the first season you get a peek at why he became this way. The reason: protection. Not for himself but for his son which quickly corrupted him but he always kept that love for his son. It has always been the one thing that kept him slightly human and gives him vulnerability that you were sure he didn't have anymore. Best episode to watch: Skin Deep.
You really don't know which side Rumpelstiltskin is on. Really he's in a plan to benefit himself in some way but, in a questionable way, he helps although you can't help wonder what his ulterior motive is. And whether it is good or bad. Who knows? This character has learnt to keep things to himself. A very clever trickster.