There should be an option, "I don't give a f*ck, I'll just do whatever I want." Most of the time there is a demon lord or simply just an evil being that is trying to destroy the world, aka killing all the humans, so I would either have fun seeing the chaos and doing whatever or I would destroy the world along the evil bastard or maybe both xD.
I see no reason to help them and if there is no way to return to my world then I why would I care to save that world? I would just play around. At most, if I found people I liked, friends, lover, etc, I would try to keep them alive but forsake the rest of the world.
It would be funny to go to the demon lord's side, say, "Hey, I'm the hero supposed to save the world, but I don't care about this world. Truth be told, I hate it. So, demon lord, let me help you out destroying it. I can give you tactical information only known to the savior of the world