Kiddushin 43 - Is Killer for Hire Liable?
Agency is inapplicable in matters of transgression, because of the principle of "The words of the Master and the words of the disciple, whose words should you obey?"
The two exceptions are misappropriation of Temple property and stealing and selling of an animal, where the sender is liable.
If one says to his agent, "Go and kill a certain person," the agent is liable for the deed, while the one who sent him is exempt. Shammai says, "the one who sent him is liable" based on "...you have killed him with the sword of the Ammonites."