My objection is many sided, and in the end comes down to questions of knowability.
A person who tells someone about an affair hoping C will kill B is ussually taking a shot in the dark- most people when they find out about affairs do not kill anyone involved. there is always the possibility that B& C have an open relationship, or c may simply divorce B and try and get revenge in court. on the other claw, if we can know A's intent towards B via C, then the whole social dynamic is changed- can A know how C will react if intentions and the secret machenizations of the human mind are that knowable? But for teh world we currrently live in they simply are not.
So if A tells C about the affair hoping to get B killed with a less than 1% chance of success, but C does in fact decide to kill B by using a gun with over 99% chance of success, then I do not consider A any more culpable than D, who entered into the affair knowing there was a chance that B could wind up dead. (not to mention b's culpability, since it was B and C who had a relationship that was defined by them, B knew the rules, could have taked it over with C, knew C's temper far better than A did, and still snuck arround on C.) Most of all I think the lesson we can take away from this is that letters should not date.
But for the world we live in A has no more responibility for C's actions than a person who throws a snowball over a balcony atop a mountain hoping it will form a giant snowball and crush the neighbor's cat. They may have acted with a certain intent, but the acomplishment of that intent via those actions is almost certainly coincidental. Unless they live in a universe where road runners say "meep meep" and coyotes wear bibs...