Yes Snape was a death eater and he did go to Dumbledore to save just Lily and he is horrible to Harry because he hated his father but just listen to the reasons.

Snape saved Harry's life multiple times. Snape stayed devoted to Lily even after her death.
Snape changes sides before Voldemort's downfall at high personal risk, as said by Dumbledore. Snape does kill Dumbledore but only on his orders.