Why did Samantha Power resign?

I am astonished that Samantha Power resigned from the Obama campaign and that Obama let her resign. She is an extremely intelligent person and was a major asset to his campaign. She is a Harvard Professor who had received the 2003 Pulitzer Prize.

Ok, the word "monster" for Hillary was inappropriate, but she apologized and retracted what she had said. There was absolutely no reason for her to leave the Obama campaign. And, in my opinion, it is totally inappropriate that Barack accepted her resignation. Apparently, there were demands from the Clinton campaign that she resigns because of her "monster" comment. But the Clinton campaign should be the last to make such demands. Why did n't Bill Clinton resign from the Hillary Campaign after playing the race game against Barack in South Carolina?

It is about time that Barack REALLY stands up to Hillary. Allowing a great foreign policy adviser, like Samantha Power, to be scapegoated is very wrong. He should ask her to come back to his campaign. I hope he does the right thing and he asks her to get back on board. Again, Barack has to REALLY fight and show strength if he wants to be the nominee.