It could be a nice improvement to have killOp report whether the op given was actually killed. Although clients couldn't use this for error checking (it's always possible for the operation to end on its own before the killOp is processed), it could be useful if someone is accidentally passing nonsense op ids.

If this behavior is added, we shouldn't report failure by returning ok: 0, as this could break client code which uses a driver "runCommand" function. Instead, we should follow the pattern used by killCursors, which is to always return success, but include an additional field about whether or not the operation/cursor was actually found and killed.