Lord Nixus has his reasons. Without giving too much away here.

1. He fell in love long ago.
2. His mother kept a secret from him.
3. Both one and two drove him into slight madness.
4. In order to gain more power He had to take it from other witches and wizards. 
5. He started the fall of magic. 

As for reasons for being loyal, he can manipulate people's feelings and cause them to feel something for him.