I have trouble with "mortal sin"? Who would do something with full knowledge it means hell forever?
Human stubbornness can be a choice of the will. We sometimes refuse to admit we are wrong even when we know we are wrong. We sometimes choose things that we know are bad for us out of our pride, ego, or laziness.
The devil and all the fallen angels, without the disordered desires of Original Sin and with the perfection of heaven before them, freely chose to reject God.
I once heard a story about a man who lived a gravely immoral life. He died and saw a throng of people all walking in the same direction. He stopped one of them and asked what was happening. The person responded that they were on their way to the throne of God to sing Psalms and praises as they do regularly. The dead man exclaimed, "Wow, that sounds awfully boring but I can't believe I made it to heaven!" The other person responded, "An eternity of praising God is heaven for us, but it sounds like it's going to be hell for you."
The basic idea behind that old story is that those who are in hell would hate heaven. The ideas of sacrificial love, gentle mercy, and enduring peace are ultimately abhorrent to them because they never learned to appreciate them in life.