how did keller explain his attitude toward steve ?

Joe Keller is in complete denial over Steven. Joe knows that it was his own fault for ordering the faulty parts sent yet he is complacent to let Steve rot in jail. Joe insists, as he did in court that Steven was to blame for sending these parts. In act one Joe says,

" So he takes out his tools and he… covers over the cracks. All right… that's bad, it's wrong, but that's what a little man does. If I could have gone in that day I'd a told him – junk 'em, Steve, we can afford it."

This is of course the opposite of what actually happened but Joe has become complacent in his denials lying to pretty much everybody.