Darius is an extreme sadist who gets a burst of adrenaline upon killing someone with his axe. If his axe lands a killing blow, he becomes extremely excited and will push his body to pull such a feat again. If he fails to land a killing blow, he loses his high and the excitement of killing disappears.
Why is Shen the ninja, a TANK?
Shen was trained to be completely devoid of emotion. Everything from enduring extreme physical pain, to psychological torture all for the title Eye of Twilight. He is unable to feel pain, and he only knows the steps of Balance, as dictated by Ionian Law.
How come Karthus can't cast Requiem all the time?