Why should he help teammates by risking his own health. They all have free roles right now. Even in good health there's little he could do for Majka, nothing for Sagan and he maybe could drag Kreuziger to 2 places higher on GC. Its better for the team that he abandon's and gives the team a final hurrah at the Vuelta.
Then there's simple reasons why Contador doesn't have much of a history of helping teammates. He's basically always the team leader since he was 24, when he first won the Tour. Nibali otoh wasn't the top dog on his team until later in his career. Also, Contador has a higher base level, and is basically the best of his team in almost any race he rides. Add to that that he barely rides one day races, and you have a rider who almost never has to work for teammates, because he's always the team leader, and rightly so. The only example to the contrary is the 2013 Tour, but we don't know how much better Kreuziger was.
You can endlessly theorize about what happened yesterday, with the attack first, and the abandon later. Maybe he wanted to give it a last hurrah, maybe he wanted to test how he was. Maybe he could finish, but only just that.
And I think that by now we have a pretty good idea that the first crash was largely his own fault. Riders make mistakes, especially when they're nervous. But I've never seen that a rider gets so much criticism for one crash. Even Kruijswijk, who threw away a Giro victory that was almost certain. The 2nd crash, which was 100% not his fault, may have done even more damage, we'll likely never know. But that's a little inconvenient when you're just out to bash a rider.