[Manga Spoilers] Hesitation

It was shown in the latest chapter that Reiner accused Marcel Galliard of hesitating to kill the island devils. I do not understand why he accused or thought Marcel was hesitant. Marcel merely said "There's no going back now. We're going to the wall tomorrow, and..." Is there something in that line of dialogue that implies hesitation, that caused Reiner to think of him as hesitating? Could someone please help me understand?