When he became a death eater, James and Sirius had seen an opportunity, which was short-lived since he worked for the ministry and was somehow higher ranked than them due to his potion’s mastery. When the war was over, Snivellus received from Amelia complete immunity and refused to listen to any of the aurors, who had fought for the order and wanted to see the man imprisoned. It was like another slap on the face for the order. The next thing was that he retired from his position in the ministry and started working in Hogwarts with a standard contract from the board. Albus tried to force Snivellus to sign one of his contracts, but it was too late, because Snape signed a standard contract from the board, which gave him the position as potion master and head of house of Slytherin. Sirius wished that Snape signed one of the headmaster’s contracts, in order to have control over that man and put him into his place, but it failed, because they were too late.