I can totally understand why this situation smells bad. However, I don't think there's one side to blame really.
If the reports are true, then he got the deal offer weeks back and didn't sign it. What means he was either considering his options or it was a negotiation tactic.
If it's the former, would you blame the club for considering other options as well?
If it's the latter, well it's a risky tactic because in the last few weeks his market value dropped significantly so Dave really has only himself to blame.
Probably it's both.
I don't understand what would you guys expect to be the right move from the club, just wait endlessly for De Gea to sign it? Of course each deal has an expiration date. The only grey area is maybe this should be on the deal. But how do we know it wasn't? It's all speculation.
On another note, I can totally understand why there isn't any comment from the club on this situation, I'd imagine there will be one tweet to thank him for his services - once he finds himself a new club. I see nothing wrong with that.