Sirius despised those muggleborns and muggle-raised halfbloods, who were supporting the dark and their ideals. He had seen it with Snivellus, Rachel and a great number of Ravenclaw halfbloods and muggleborns. Snivellus was also a Slytherin muggle-raised halfblood and that was the reason, why James, Remus and he were pranking him, in order to stop him to corrupt James’ love of his life with the dark ideals and traditions. But their pranking wasn’t successful and Severus broke up the friendship between Lily and him. From there Lily lost everything, because later it was revealed that Albus had plans for Severus, but Lily destroyed those plans and the man went to his own path, where Albus had no control. They tried to prank him a few times, but there was no point and reason, because he was out.