This sidebar says that the Catholic belief is that angels have a will, but it is unlike human free will. We can change our minds, back and forth, again and again. Angels, apparently, once they choose, cannot change their minds.
And this makes no sense. They're using it to, apparently, explain how an angel could disobey God. Not just one angel, but thousands (millions?) of them.
I was always raised to believe that angels have no will, they have no choice but to obey God, which is why the rebellion of Satan never made any sense to me. It's always made more sense in the (and this is just my understanding of it) Jewish stance on it, which is that "Satan" is just an angel fulfilling a purpose. A guy doing his job.
After all, if there was no evil, what good would free will do us? We'd have no options? Obey God or obey God? With Satan tempting us, we can exercise the free will that God made a point of creating us with.
And even this explanation makes little sense to me. If an angel can never change his mind, then the original decision to obey God should still hold. None of the angels should have been able to make another choice.
Maybe I'm missing something, but it just doesn't make sense to me.