It's also worth noting that Nahiri had came directly from Zendkiar, and was concerned about Sorin and Ugin. The enchantment on the eye of Ugin was designed to immediately notify Sorin and Ugin if the seal was breaking. When Sorin didn't show, she thought her mentor could be dead and went to Innistrad to confirm it, only to find Sorin alive much to her relief.
The alarm didn't reach Sorin as either the Helavault soaked it up or he was unconscious from the strain of creating Avacyn.
Nahari stated he had to hold up his end of the bargain, as it was her world she was sacrificing to protect the multiverse. Sorin was dismissive of these obligations, and never at any point apologized or tried to correct his failings.
It's silly to expect someone to be perfectly fine with being betrayed given the sacrifices Nahiri made. If you made a deal to burn a forest on your land down in exchange for compensation only to receive nothing, you would be pissed off to an immense degree.