Ah, but Argus Drell isn't your typical assassin. He used to be when he was younger, and working for the government, and later on for independent contractors. If he thought you'd interfere with his mission, he'd gun you down without blinking.
Now...well, if you intend to stop him, he'll still gun you down without blinking, but he's guided by a very strict code of morals. He'll at least ask what you want. Unless you're a slaver or some other form of scum. Then he'll happily shoot you, strangle you, snap your neck, plant a bomb, throw you off a tall building, or some other fun form of execution.