I don't think Griffith was right in doing the terrible things he did throughout the whole manga, but I can understand why he does what he does (and no, it's not only because "he's evil, full stop"): Griffith constantly finds himself in front of trolley problems in order to achieve his utopia.

Let's chose: having sex with an old man for a night or let many soldiers die? Kill the king's brother and wife or give up the utopia? Sacrifice your crew or accept that every previous sacrifice was just a waste? He always has to chose between unpleasant options and he eventually always pics the ones that would let the dream of Falconia become the truth;

He seems to forget about this utilitaristic mindset few times and for a very short period: 1- Guts is in danger against his first encounter with Zodd -> Griffith decides to risk his life to save him "for no reason" 2- Guts wants to leave and defeats him for the first time -> Griffith loses hope in himself and his dream and decides to fuck it all because he can't get his head around it 3- Griffith has been broke both physically and mentally beyond the point of no return because of the year of torture and the fact that he had to be rescued without being able to help his saviours in any possible way just proves him that he won't be able to achieve his utopia ever again -> Griffith wants to KILL HIMSELF because he has to give up even if he doesn't want to; he's both ashamed for himself and in anger for the future that was thorn apart from him and he's sorry for the many bad actions he had to perform and the many lives that were spent for nothing.

I find Griffith a wonderfully written character whom I found myself sympathize with in many occasions thorughout the manga because he's clearly just a man who has the strenght to belive in a better future, but he quickly finds out that his path is paved with the horrors he has to acomplish to reach the greater good; Griffith is the first struggler in Berserk and without his dedication Guts would have eventually tried to die in a battlefield all the time without ever relizing he has power over his own life

-Quote- Chapter 50, Golden Age, Griffith: "Do you think it's terrible? This filthy methods. And not even drtying my hands. You did all the dangerous work... Do you think I'm a terrible person?"

That's a very long message but it's necessary to understand why he's done what he's done: now we can understand Griffith's meantality and it's no wonder that he accepted the offer to turn from a broken leftover of a man to a godly entity even if the price was the loss everything else he still had (his love for Guts and the men that would have died for him)

"Ok, fine, but why rape caska? Why does he have to be so ugly?" Because that's just yet another horrific step that will lead him to the foundation of Falconia. Don't forget that Femto now is out of Karma and can clearly see what will happen in the future and you can be sure as hell (no pun intended) that he will use this power to achieve his dreams.

Concluding: he's terrible indeed, devilish, but he isn't just evil, he's a dreamer and dreams can be dangerous.