Lucifer was the closest creature to God before his fall. He knew everything a creature could know about God, about the mysteries of creation, how it would work. And yet he still rebelled. He knew beyond doubt, and he still rebelled. There is no coming back from that, Lucifer wouldn't repent, it's not in the card for him because of the nature of his own being and the consequences of his decision aeons ago.
The devil deserves no sympathy. And he knows he's going to lose eventually, yet he keeps going. Why ? Because of his hatred. His hatred for man.
Just like Jesus would still have died on the cross if only it meant saving one soul, Satan would not hold back because if all his efforts lead to the perdition of just one soul, it would also be worth it for him.
He's not human, it's not in his nature to repent. He cannot go back to God like we do. Which makes hate us even more.
Mark Twain is wrong and misguided in his sympathy for the devil. He humanizes Satan, a gravest error.