“You'll be cut from humanity, and the evilness will possess your mind.” Ubrik, Vol. 3 Ch. 2

Some say Femto is Griffith’s true spiritual form, but I disagree. Everything that was human about Griffith’s human soul disintegrated during the transformation. The real meaning of the ritual is less about the physical act of sacrifice, and more about the symbolic sacrifice of your own humanity. It serves to sever ties with the human race. Of course, in order to sacrifice your humanity, you must first have it, and once you lose it, you’re not yourself anymore. You're an empty vessel which, according to Ubrik, gets taken over by "evilness." Therefore, Griffith != Femto.

“Everyone’s dead. I’m through it... I want... to die... Never to think... Never to feel.” Griffith, Vol. 13 Ch. 3

Griffith was tricked into believing his choice would make him feel better, but it only made things worse... much worse. He was confronted by the tears he froze to protect himself, and how meaningless it was now. He didn't like seeing his friends slaughtered. I can't say if he regretted his decision, but he didn't want to deal with it anymore. He couldn't face their pity before, and he couldn't stomach their horror now. He just wanted to die. To shut off his mind and his heart, close his eyes and never wake up. Femto became the sterile, callous personification of not his dream, but his ambition.