Well, Dumbledore wasn't completely honest. He mentions Severus's abusive parents and Severus joining up with his houesemates who became Death Eaters.

But Severus was best friends with Lily for the first five years at Hogwarts. And he was bullied by James Potter and the Marauders for being Slytherin and for being friends with the girl James fancied. Dumbledore watched Severus being bullied and being pushed into darkness by his favourites without doing anything. He even let Sirius Black go unpunished when he tried to feed Severus to a werewolf.

So Dumbledore lied through omission in his talk to Severus. Severus didn't go dark in his first year. No, Dumbledore gave up on any student as soon as they were sorted into Slytherin. 

Dumbledore failed Severus. He was only a student when he was bullied and almost killed by his class-mates. And even now Dumbledore doesn't see his role in Severus turning to the 'Dark Side'?

It really shows how low Severus's self-esteem is, that he accepted this half-arsed and dishonest apology. Dumbledore used him and watched him being bullied. Sorry, but Dumbledore completely failed doing his job as teacher and headmaster. He only was the greatest Headmaster of Hogwarts when Gryffindor house was concerned.