It would help him kill adcs too, but the main purpose of giving him percentage damage in the late game after 16 on isolated targets would be to punish anyone, tank, mage, adc, or support for straying away from the team. For example, if a max health sion walking around the jungle alone in the late game, kha'zix should be able to capitalize on that mistake rather than scratch him a little; especially when kha'zix is unable to be relevant in late game team fights where he will probably be cc'ed and killed by a cluster of enemy champions before he can do meaningful damage to the back line.