Here's the thing with Vegeta: All he wanted to do was fight Goku. Fighting Goku is his goddamn "rosebud." Its largely the reason he gets up and trains every day. Finally Goku comes back from the afterlife, but its only for a day, and then all this bullshit happens. How ridiculously frustrating is this for Vegeta? The answer: very ridiculously frustrating. So much so that the fate of the damn planet takes a backseat to it all and Vegeta does the Majin thing just to finally get his chance to fight Goku. But as they're fighting, Goku brings up Bulma and Trunks and the danger they're in, and Vegeta suddenly realizes how foolish this all was. He feels like an idiot in some ways. But, fuck man, he just wants to prove himself against this "lower born" saiyan and shit keeps getting in the way. Its eating at him, perhaps killing him in some ways. He figures that if he can't get his shot to top Goku, and it looks like he never will, then at least he can knock Goku out and fill his shoes. If defeating Goku isn't an option then maybe at least showing him that he can save the earth too is. It'll be a little bit of closure at least.

Is it selfish? Yes. But thats a bit too black and white of a word for my tastes. Vegeta's wrestling with a lot of things and this is his prideful and arrogant way of trying to make up for past mistakes, and dealing with the fact that he, the former top dog, is no longer top dog. I would say his motives definitely lean more towards "good" but Vegeta is, IMHO, one of the most nuanced characters in the series so you're never going to get just a straight up black and white answer with him. Just my 3 cents right there.