>>40734754 (OP)Active imagination. Sit or lay in your room with eyes closed and imagine you can do truly anything you want.
Ponder on the most atrocious things people have done in human history and take their place. Try to understand them and fulfill their desires through the actions you do in the dream state.
Or, for a more "guided" approach, focus on something that really upsets you or makes you angry. Think of times people betrayed you, ridiculed you. You need to engage with something that will resemble "self-hatred", because if you can't easily access the dark side now its because you are projecting it outward and it manifests as an enemy while in reality its another part of you and your wishes.
Turn that self-hatred into a hatred for the world nothing can stop. You even see in the fight with Grunbeld for example, Guts is bleeding his organs out, his bones are all cracked and his limbs are facing in all directions, he still has one thing on his mind and nothing else: destroy, revenge.
Do this for too long however, you will literally become sick as a psychosomatic consequence.