The person who tells him to kill is worse because they are the one with the evil intent and they are doing more than just getting someone killed. They are getting a person killed and corrupting someone at the same time which is way worse than just being the one who got manipulated.
A lot of people tell others to kill, but they don't do it. Terrorists encourage people to be suicide bombers, but the blood is on the hands of the people who actually commit the crime. Obviously, it's wrong to tell others to kill. But the actuall act is much worse.
It is easy enough to tell another person to do this and that, since the person ordering the other around is unlikely to fully understand the consequences, or even have any proper feelings of guilt or unease. It will almost be, as if what they have ordered didn't happen- there would be no, if any, feelings of guilt because they themselves did not do anything. The person doing the killing, however will have gone through a lot of moral conflicts and still decided to continue with the killing. They know what it means to kill a person, to remove another's life.