First of all, I was confused by the fact the Snape was Voldemort's most trusted servant, while at the same time he was a half blood. Voldemort's whole regime is that half blood's and muggle borns should be killed. Voldemort's father happened to be a muggle, as was Snape's father. I think this might be the only reason Snape is almost considered a friend of Lord Voldemort, who has no friends.
Another thing I noticed, after reading the book after this one, is that Snape's rudeness towards Harry was planned. Dumbledore ordered Snape to protect Harry, but not make it too obvious. Snape has to be Harry's greatest enemy other than Voldemort. This is because of a grudge he had against Harry's dad. The twist in this was that Snape was in love with Harry's mom, so he feels he needs to protect him.
The last thing I noticed after reading this book again, is that Snape brushes things that hurt him away. For example, I know he loved Dumbledore a lot. Dumbledore saved him, and cared for him. So when he killed Dumbledore, he did it really quickly. No suspense. It was planned, but he couldn't bare to see a good friend dying at his own wand. So he did it as quick as possible. He wanted to make sure there weren't a lot of memories.