@Qelix, there's no trigger for his power to work, he can just "end" something he wanted to stop.
He only draws the line by setting any killing intent towards him his target.
His ability is basically a death to all thing that has a concept of living. Even if it is a machine. If it has concept of alive like cars that can move. He can set that function to die.
That's why in earlier chapter, he can set only a particular body part to 'die'.
He is basically a death God.
Also he don't really need to speak to activate the skill. If he thinks about it, the target will die.
He's not really interrsted in killing people tho.