Pete gave him so much. So much so willingly. And Vegas is keenly aware of it. Companionship, love, trust, power. All given without reservation. Even a monster like Vegas can be moved by that. Not because Pete was some kind of paragon of goodness, in fact, because he was monstrous, too. The like in them called to each other.

Well, and Pete was very aware of Vegas' many insecurities. He knew power, or the appearance thereof, was important to him. He knew and willingly, happily gave Vegas power: power over him in their bedroom, power to avenge this wrong done to him and their friend. Pete truly understood him. And Vegas met that understanding with his own.

"I won't forgive anyone who hurts you again."