Gellert Grindelwald
He had loved Albus, in the sense that he had found (or thought he found) a co-conspirator in his grand plan, who was brilliant and eager and adoring. He loved Albus, but it was undeniable that he loved power more.
He loved the power and the thrill of murder and the Greater Good, and at the time, it was more important than a silly romance.
But when Albus finally confronted him, with his blazing eyes and fierceness, he wondered if he had made the right choice.
And when Voldemort comes for him, he knows he didn't, and he welcomes death.