I agree, but there can be a situation I believe where both can be convicted.
If Bob actually believed that killing this person was a necessity, then both can be convicted.
1) We can establish that Person X knew Bob would do it.
2) Bob should have been smart enough to not just trust a dear friend's word on something that involved killing someone.
So, to say who is at fault would require knowing some depth of the situation and establishing free will as well as if Bob was 'close' to person X.
Example: I tell a random stranger to go kill Bob and the stranger does it. I don't think you could hold me at fault since the stranger was dumb enough to actually do something I said, especially if there is no intent.