What is Dumbledore's dark past?

As a teenager in Godric's Hollow, Albus Dumbledore met Gellert Grindelwald — a brilliant, charismatic, and dangerous young wizard who had been expelled from Durmstrang. For a summer, they were inseparable. They shared a vision of wizards ruling over Muggles "for their own good" — what they called the Greater Good. Dumbledore was brilliant and grieving (his mother had just died) and Grindelwald was the first intellectual equal he'd ever met.

How did their relationship end?

Catastrophically. Dumbledore's younger brother Aberforth confronted them over neglecting their sister Ariana — a magically traumatized girl who couldn't control her powers. The confrontation became a three-way duel. Ariana was killed in the crossfire. Nobody knew whose spell killed her — possibly Aberforth's, possibly Grindelwald's, possibly Dumbledore's.

Grindelwald fled. Dumbledore never forgave himself.

How did this shape Dumbledore's entire character?

He spent the next decades refusing to confront Grindelwald even as Grindelwald rose to power across Europe — because he was afraid. Afraid he might be tempted back to that old philosophy. Afraid he might discover he had killed his sister. His refusal to act cost lives. When he finally fought Grindelwald (1945), it was the greatest duel of the century. He won. Then he put Grindelwald in Nurmengard — his own prison.

What does this history reveal about Dumbledore's manipulation of Harry?

That it comes from a man who has his own complicated relationship with power and sacrifice. When he guides Harry toward voluntary death, he is, in a sense, asking Harry to do what he couldn't do for Grindelwald — sacrifice everything for the greater good without corruption.

Why did Dumbledore never tell Harry about his past?

Partly shame, partly strategy. He believed that Harry needed to trust him, and the truth was complicated. Hermione's criticism in the final book — "He has always kept too many secrets" — lands. It's the most honest assessment of Dumbledore in the series.