In the BBC series "Sherlock", why did Moriarty kill himself?

A) He felt bad for what he did.

B) He knew Sherlock would win and decided that his life was meaningless if he couldn't beat Sherlock.

C) He knew that only he could call off the assassins prepared to kill Sherlock's friends, so he killed himself so Sherlock would have only one way to save them.

D) He knew he would be sentenced to life in prison, and not wanting to endure that, he killed himself.