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Did Thanos Do Anything Wrong?
He sacrificed his 'daughter'.
Thanos could have sacrificed any soul to obtain the Infinity Stone, but he chose his own adopted daughter. To kill a family member is considered more ruthless than killing a stranger.
One can argue this action is what makes him more 'heroic'. He had everything to lose by sacrificing the only soul in the universe he cared for and loved - and not much to gain personally.