What do you think was the worst thing Snape did in the books?
Even though he was not able to do it (he was stopped by the trio), taking Sirius to receive the dementor's kiss was I think the worst thing he could have done. Destroying someone's soul I think is the most evil thing anyone could do
Not listening to Lily after she told him to stay away from the Dark Arts and also wanting to impress her by becoming a Death Eater. Lily would have chosen Snape as her husband instead of James if Severus listening to her.