Mao is obsessed with C.C., and fair enough; he got his geass when he was just a child and was both raised by her and saw her as the only real person.
He wants to take her away to an isolated location. But he's ax crazy.
Lelouch entraps him and kills him.
Lelouch learns the difference between "shoot him" and "kill him" when Mao turns out to be less dead than advertised. He kidnaps Nunnaly to force Lelouch into a confrontation.
Suzaku has his back and helps rescue Nunnaly, but Mao reveals his dark secret: he killed his own father.