Boris Skullcrusher

I am a bit confused with Boris Skullcrusher ability

It says:

"Destroy target ally with cost 4 or less"

Ok to me that says that kill an ally with 4 or less health

However sometimes it does not give me the option even when I have 10 shadow points and an ally with one health. There are no items or effects on that ally that would stop me from killing that card.

The card in question is Plasma Behemoth ( Yes I know it has 5 health but at the time it had 1 health)

Also will a card like Infernal Gargoyle be destroyed by Boirs' ability or because it has -1 damage it really has 5 health and the ability does not work on the card?

"Destroy target ally with cost 4 or less"

Plasma Behemoth is cost 5 and thus will not be killed. Health does not matter.

Infernal Gargoyle is cost 3 so it can be targeted and killed with boris.

Oh it is a question of recourse cost not health

Ok thanks for that