I don't know about that. Dark Magic, as I see it, is magic that involves harming/killing of another person. Legilimency, though gives access to the thoughts of the victim, doesn't really harm the person, unless they have something to hide. ;) So, I don't think that is considered a Dark Art.
I think Legilimency is considered a Dark Art - given that it gives the Legilimens access to the thoughts which the victim may want to keep private, and doesn't really give the victim the opportunity to repel the attack, and he uses that just fine.
Just because he uses Dark Magic for the cause of the people doesn't make it good. The fact remains that he
To defeat Grindelwald, yes I think he would. Dumbledore would not be using it as a dark wizard, he would be doing it to save a great number of people. I think that Dumbledore would know the benefits of his actions, and not linger on the fact that he had to use dark magic.