Does anyone know the point in Snapes life when he become teacher at hogwarts?

Snape 1st applied to be a teacher because Voldemort asked him to so he could spy on Dumbledore, however he did not get the job. When Snape later found out Voldemort was targeting the Potters, he approached Dumbledore and asked him to save Lily's life. Dumbledore agreed to help, in exchange for Snape spying on Voldemort for the Order. So Dumbledore gave him the job of Potions Master (instead of the one Snape applied for: the DADA position because it was cursed). This way Voldemort would think Snape was spying on Dumbledore, but in reality he was spying on Voldemort for the Order. Snape was working as a teacher and spying for the order for over a Jahr before Lily was killed and Voldemort disappeared. I don't think Snape stayed on to avoid going to Azkaban. He had enough evidence and support from Dumbledore to Zeigen the wizengamot that he was working for the light. He continued to teach at Hogwarts because both he and Dumbledore knew that Voldemort was still alive and would return. Snape stayed so that when Voldemort returned he would be able to resume his role as a spy, only this time it was to protect Harry. It was so Lily's sacrifice was not in vain.

Severus's got so much grieve, that he doesn't know what to do anymore because he's Lost Lily so he go's to Hogwarts and does a favour for Dumbledore.