Being a product of your environment/being oppressed by the world isn't a flaw, it's a sob story to excuse her actions.
The definition of a mary sue is someone who is loved by everyone, except for the enemy. When she killed Sasha, the intention in the story was to make the reader reconsider who the good guys really were. The fact that half the cast is against Eren the villain and in support of Gabi the self-righteous vigilante speaks volumes of how much of a mary sue she is.
100% false. A Mary Sue is "a type of female character who is depicted as unrealistically lacking in flaws or weaknesses." It has nothing to do with how opposing characters view them.
When she killed Sasha no such thing was done. If the intention was as you said, it was executed incredibly poorly, and we already know that Isayama is too good of a writer to fall into such a large pitfall as that. It was a war and sometimes not everybody makes it back. Sadly that somebody who didn't was Sasha.
And "half the cast is against Eren" because he's been AWOL and now he's revealed that he intends to wipe out humanity beyond Paradis Island.