Why would Gamora expect Thanos to care about anyone especially since she's been witnessing him torture people her whole life? Even her own sister? Her laughing at Thanos at that moment made total sense. In her mind he failed because she could never fathom him loving anybody.
You make it seem easy that you would be able to kill your own mother, brother, sister, lover, child. I'll tell you one thing, If I had to chooose killing the universe or killing my loved one. I would never be able to kill somebody and live with myself. So hopefully I don't ever have to deal with it. Maybe you got a cold heart and can do it, but I guarantee you 99% of the people couldn't do it. So let's just hope that the universe's existence doesn't count on me.
Maybe if I had to let somebody die I could turn away and then go commit suicide because I would be way overthrown with guilt, but to actually pull the trigger and murder somebody myself no way I'd be able to do it.
Dr Strange has to protect the Sanctum Sanctorums from other dimensional threats. He can't abandon his post forever since he is the most powerful sorcerer on Earth.
Gamora and Nebula made up in GotG2. That was character development, she spent her whole life trying to survive at the expense of her sister getting tortured, she couldn't take watching her get tortured anymore, as she realized that Nebula actually did love her.
