Don't underestimate the importance of morale. Humans aren't robots, after all. In both hypotheticals, every single messy death would be accompanied by more of the horde breaking away to cower on the edge of the arena.

They were told they'd be killed if they didn't attack? Compared to the sheer, pants-shitting terror of the threat currently towering in front of them, that potential future consequence won't seem near so important as "must get away".