My first ever rat was an unexpected rescue. His old owners kept him in a plastic storage bin by himself in their garage because apparently the wife didn't want a rat in the house. I don't know how long he had been living way. He was overfed and absolutely socially neglected.
I named him Lestat. My first few weeks of having him, I could tell he wanted to be near me, but whenever he had the chance he would bite. Not just nibble, but would draw blood. But it wasn't out of aggression, it was out of fear.
So I created a playpen and would sit in that playpen for HOURS with him, every single day. I got on ratforums.com and collected as much info as I could. A week later I was about to get him another male rat (I wasn't financially ready to take on a rat at the time, admittedly, but I did the best I could) named Louis. Louis was far more sociable and Lestat was able to watch and learn from him.
A few weeks later of working with him every day, Lestat got friendlier. Now he'll run and jump into my lap and snuggle. I have no fear of him biting and now he's my little bestest buddy.
So please don't ever give up on a rat that seems... a little off. Maybe all they need is a little chance. Or... several little chances. :P