This is how Snape has always talked to Harry, teaching him non-aggression accompanied by insults and loathing. Snape has just cast an Unforgivable Curse and knows how bitterly it cost him. Harry wouldn’t be able to do it based on vengeance alone; it took Snape greater emotions than that to pull it off, and it ripped his soul apart. Everyone deserves, if at all possible, to remain too innocent to cast an Unforgivable.
