That's a questionable point, whether it is ethically right to be an "asshole" through your career and then donate it all once it ends.

I would say it (up to a point) is right. Donating/helping early in your life has a low impact, so the main reason I think it's reasonable to argue that way is that not everyone will donate at the end of their lives. So by gaining leverage early on (not contributing to philanthropy, not helping, etc) you will compensate for those who wouldn't donate and help make the world a better place once they got rich.

If you look a the billionaires out there quite a few will sit on their growing pile of cash until they die.