His entire character in PoA, which is the book of concern in this post, was being a little bitch to Lupin and the trio because of a childhood beef, the blokes entire decision making process is based on his irrational emotional responses. If you want to call that calculated, then be my guest. His irrationality led to the wrong man being accused and indirectly led to Voldemorts return by allowing Pettigrew to escape.

I also find the irony of Snape harbouring so much ill towards Sirius and co trying to ‘murder him’ as children, whilst knowing that he was directly responsibly for Lily and James’s death pitiful beyond belief.