Nate was a bastard to the very end...man..what a bitter character he was...even in death he was cruel. At least they softened him up in the last 5 mins.
Whoa, you're missing the forest for the trees. All the ghosts on SFU are a reflection of the living person's inner thoughts. They are a device to show the audience a conflict going on inside each character without telling other characters.
Nate acted like a bastard to Brenda, because Brenda is deeply insecure and cynical. He's the little devil on her shoulder tempting her to despair over Willa.
Notice how he was so supportive of Claire? For her, he was the little angel.