How we cite our quotes:
Deep silence had fallen with the starless dusk, and they leaned on each other without speaking; but at every step of their climb Ethan said to himself: "It's the last time we'll ever walk together." (9.127)
Ethan's thought takes on bitter irony when we see that after that night Mattie will never walk again. This is a classic example of the tricky foreshadowing that Wharton layers into the tale.
"I'm not crazy; but I will be if I leave you." (9.149)
Maybe Mattie was already suicidal, but we don't have any evidence of it. We think that her desperation, love, fear, and the prospect of being alone in the world again push her to this terrible transformation.
"We can fetch it; I know we can fetch it-" (9.171)
Ethan is talking about hitting the elm tree. Some of Ethan's previous thoughts and whisperings could be taken as suicidal, but there is a big difference between thought and action. Ethan, in this moment, seems thoroughly transformed.