I have savage strike and I used "set decapitationchance to 100", but I still can't decapitate anyone. How do I do it?

When you do a 'power strike' with either one handed or two....you need to make sure you're standing still while swinging. You also need to make sure that this swing will be a killing blow. This should net you better chances of landing the decapitation.

dont u have to choose one of the perks to be able to decapitate? like, a specific 'decapitation perk'?????

Yup it's one of the perks in 2-handed skill, not sure about 1-handed. ("chance to decapitate") >> this proves it doesn't happen all the time, it's just a "chance", at least that's what I think.

I was shocked when I was fighting some boss with an axe and he took my Khajiit's head off. I honestly didn't know they would do that. I thought to myself, "maybe he will make a rug out of me after all".

In "One Handed" combat this is an unlockeable perk for use with Bladed weapons.

Azrael
The Nord with the Sword