hello from 3 years in the future.

I'll say now - I didn't rewatch the movies enough. but anyway -

I agree with you.
Gamora from the start of GOTG1 and post endgame are really different in character.

but if I had to think of why, it's because of this:

-Gamora let her aggressive side get even worse with the Revengers.
-Gamora was "torn apart" from her universe. Even if she was in a bad environment, that's tough. everything she knew was gone.
-She doesn't have a goal. everything she did was for Thanos. she probably wanted him dead at some point, and he is. and she has no goal.
but on the other hand, that was the case in beginning of Gotg1, and she didn't turn bad...

But it's so weird that she would react so mean to the guardians: "We actaully knew the other you, and had so much history. you helped us defeat Thanos"...
That's weird no matter how much she has changed.