They've already scouted ahead before going to Rosymorn, so Gale has a fresh new suicide mission and Nym is struggling with the fact he seemingly cannot convince the wizard this is a stupid waste of his life aaaand the realisation that he's terribly and desperately in love with him.

And crucially, he hasn't been back to the underdark in this timeline, so he's never confronted his trauma and he's never had positive experiences with the myconids or the gnomes. His empathy is kind of at an all time low.

When they come across the githyanki training grounds and it hits a little too close to home, he writes the whole place off. He talks to as few gith as possible, and he never sees them as more than a den of very familiar wolves.

So, when they get to the Blood of Lathander and he realises what is likely to happen if they try and steal this relic (having read the accounts of the monks), he grabs that thing. And he tells the party (Lae'zel, Gale and Shadowheart) to leave him behind.

And with some drama, they do. Gale casting featherfall and dimension door really comes in clutch. The three make it out alive. And Nym waits.

After he's resurrected, Gale confronts him for his stupidity and unnecessary risk and Nym is just all. But look, we got what we wanted. A scepter of divine light to help us in the Shadowlands. A whole bunch of enemies who would hunt us down all dead. We won. Oh, it felt bad to see me die for it? Hm? Yeah I bet it did.