Did Severus Snape betray Albus Dumbledore?

Answer 1

Snape was very guarded about his past. We only got the occasional brief glimpse of what his home life may have been like. We are given the impression that Tobias was not very kind and understanding to Snape all the same.

When Harry accidentally breaks into Snape's mind in Harry Potter and the Order of the Phoenix and sees some of Snape's childhood memories, one of the memories he sees is Snape's dad yelling at Snape's "cowering" mother (USA Scholastic hardcover edition, p592). The fact that Eileen Snape was cowering from her husband suggests that he was physically abusive to her, since you don't usually cower from someone unless you are afraid they're going to hit you. If Tobias Snape did indeed beat his wife, then it is certainly possible that he beat young Severus too.