Did everyone miss the bit about Snape's life as a child. It would be completely normal for a child who is abused and who has no personal possessions to be possessive of a person that shows them attention, affection, or friendship. Snape was damaged from the beginning. He had no contact with a healthy relationship, even as a baby and child. Therefore, you cannot blame him for wanting to perpetuate unhealthy relationships as a teenager or adult. He had no idea what normal and loving were. I think it is a shame that some people don't see the tragedy of that. When you are constantly put down and picked on and have no reprieve from that in your home life, you cannot be a functioning adult with the ability to decipher right from wrong.