The Last Of Us Explained: What Does The Ending Really Mean?
4. Your Enemy Is Someone Else's Friend
One thing that allows to truly appreciate The Last of Us is that it's told from just one perspective, yet alludes to other viewpoints multiple times.
Nowhere is it more evident than when Ellie encounters David. His dubious habits and actions aside - even though it's hard not to think of them - there's no denying that he values loyalty and sacrifice above everything else. You could even turn him into a positive character if he was the protagonist of this story.
Marlene and the Fireflies are not bad people for wanting to kill Ellie - Marlene is not happy about having to sacrifice the girl in order to deliver the cure - just like Joel is not a positive character for killing in order to save her.
The judgement about positive and negative characters is dependent on the perspective, especially in the complicated setting of the game. Some want to survive no matter what, others are more concerned with remaining human. Evaluation of character is possible only after taking a stand on either side of the argument.