Love wins, Always.

From Harry Potter and the Deathly Hallows:

"His scar burned, but he was master of the pain; he felt it, yet was apart from it. He had learned to control it at last, learned to shut his mind from Voldemort, the very thing Dumbledore had wanted him to learn from Snape. Just as Voldemort had not been able to posses Harry while Harry was consumed with grief for Sirius, so his thoughts could not penetrate Harry now, while he mourned Dobby. Grief, it seemed, drove Voldemort out...though Dumbledore, of course, would have said that it was love..."

Dumbledore said that Snape was an excellent Occlumens and of course we find out at the end that he loved Lily all along. I'm guessing that his continuing love for Lily helped him keep his mask, kept Voldemort out of his head, kept everyone out. Except during the lessons when he took those thoughts out and put them in the pensieve. He opened himself up.

The first brother died because of presumption of power. The second one was consumed by loss of his love. The third willingly went and passed on the Hallow to protect someone he loved. Voldemort wanted the Hallows to rule with power. But in the end it's love that will win.

Always.