Hi,

I don't have the game in front of me, but I think the Support Card for the Blue team says something along the lines of '...a Genestealer can be killed if a skull is rolled during defending...'

Does it matter which way the Marine is facing? Can he use this ability if the swarm is behind him?

All special abilities ignore facing unless the card specifically says it does not. So yes, he can counterattack from behind.

One of the few times being surrounded is beneficial.