what is the difference between
それが人の本質だからでは
and
それが人の本質だから
saw this in an episode of attack on titan
This では is short for ではないか, which roughly means "..., isn't it?". For example, 夢では means "It's a dream, isn't it?" or just "Isn't it a dream?".
それが人の本質だからでは
= それが人の本質だからではないかIt's because that is the true nature of humans, isn't it?