I've come to understand the "Majin Seal" as a device that merely unleashed the anger and sorrow Vegeta had in his heart already. Vegeta at that point was mad because he became soft. He wasn't a killer anymore, he was a family man. With Son-Kun dead Vegeta had nothing to fight for. He lost a large part of himself. When Vegeta saw Goku fight Yaken he realized that Son's power passed SS2 Teen/Adult Gohan's and his! He was wounded, he the Prince of the mighty warrior race reduced to a "good guy!" Vegeta let the evil warlock simply unearth the hidden potential he already had.