Konohamaru could technically kill a Kage with Rasengan. You have to have a well rounded skill set to be considered Kage. I can argue that Mei could beat anyone with her corrosive mist technique. Who can avoid that?
Anyone could techniqually kill a kage with a kunai, the problem is connecting the kunai to the kage's head.
The problem with all these attacks that can kill a kage are that they require the person using it to connect it with it opponent. With jutsus like rasengan and chidori, they are dodgeable.
Techniques like C4 are not since the an opponent without chakra sensing abilities won't know its even coming.
Chibaku tensei isn't dodgeable because KM naruto can't get out of it with his speed.
There are many techniques that have the "potential" to kill kages, but the techniques we are talking about seal the deal because they can't be avoided, or the kages don't have any LOGICAL way to deal with them.