*This section will be of my own opinion of movie Folken since I have only read a synopsis and have not, unfortunately, seen the movie. I do have an Escaflowne movie art book though and will try to get information from that translated to see if I can post it here.
In my opinion, movie Folken is, in a way, mysterious because we never completely see what is behind the mask of hate, anger, sadness, and pain. We are not shown his past, and we are not shown things from his point of view. We are simply told that the reason he is so hateful was because his brother, Van, was chosen to be king, and he was not.
Now, stop and think. Maybe he becomes the way he is because he feels like people do not think he can handle the responsibility of ruling a kingdom. Maybe he feels inferior to his own family because he was not chosen. Maybe it makes him feel like something is wrong with him, which gives him the urge to prove that he is not incapable of becoming king. It does not make what he does as a result okay, but, whatever the explanation may be, it ties in with why he wants to destroy lives since he allowed his bitterness to become hate.
He starts his plan to destroy the world by killing his parents. I think that's pretty disturbing and can not say much on that particular action. I also do not want to explore a mind like that. As his father is dying, Folken tells his father that he will kill Van by his own hands. The only thing I can say to that is that I guess it is the quickest way to become king if he was not suspected of being the killer and he was very discreet about the murders.
He does not stop there though and destroys the White Dragon Clan and his country for a reason I can not speculate. Maybe it is to show people his true power.
As time passes, he adopts a seer to predict when Hitomi (the Wing Goddess and the one who will save Gaia from destruction) and Escaflowne become one. She has a secret though, which I found interesting. She is in love with Folken. She is very secretive about her emotions, but it is true. Sora stays by Folken's side till his death and stays with him even after he is dead. Why she is in love with him is beyond me since all Folken seems to be able to show is hate. It is debatable whether he likes her a little bit though because he is kind to her throughout the movie except in one sequence. And in the end, he sets her free. It is an offer she refuses just to stay with him.
I keep reading on websites that Folken was unredeemable, but I do not think that is entirely true. In the end, Hitomi stands up to him and tells him that all of Van's sadness, pain, and suffering are because of him. It's very debatable, but I think Folken begins to think about what he's done. He knows he's caused so many others pain, and when Jajuka plunges his sword into Folken's chest, Folken whispers, "Retribution." Folken knows he got what he deserved and tears the sword from his chest. He collapses to his knees, and his last memory is of Van calling out to him joyfully. "A sound I always cherished," he whispers and dies with a smile.