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.
Have I mentioned that Suzaku's father was the former Prime Minister of Japan before Britannia invaded?
It turns out his "suicide" was actually a coverup for a young Suzaku killing him.
This is why Suzaku is so reckless in battle; he's suicidal.