Guts killed and is willing to kill a lot of people for his own sake, just like Griffith. He is just in denial over the fact he is very demonic and barely any different from his arch-enemy.
Guts violated Casca while she was insane by forcefully kissing her. He is arguably worse than Griffith, taking advantage of an already mentally ill woman.

If Guts becomes willing do go to extreme lengths, to lose himself in his rage and anger in order to try and kill Griffith, he will just become an "useful idiot"(See Ganishka) that will serve his enemy by driving people towards Griffith's arms.

Will Guts end up like Ganishka? Or will he give up his revenge and try to help Griffith, his former friend who spared and saved his and Casca's life several times, recover his humanity?