On the contrary the third act and ending was quite well done. A lot of people didn't quite get the ending but the movie has already spent quite some time building diana's character and the whole point of the final act was not supposed to be a super saiyan epic battle between ww and ares, rather a test of diana's character when put into that kind of situation and given a choice to decide humanity's fate


Diana is zeus's daughter, hence why she calls him brother

WW did what she did because of her love for steve, indirectly to say, she did it out of love

It's something the director left audiences with to connect the dots. It's sprinkled all over the film, like that scene when diana looks at a german woman's dress and without further exposition, cuts straight to the scene when she enters the ballroom with a new dress. The film trusts you to connect the dots yourself

Anyways it's not hard to grasp. Steve trevor wanted to save humanity even at the cost of his own life. Loving steve means honoring his sacrifice, his memory and his ideals. And not killing dr. poison was her first step towards this goal. And not going along with ares's plan to destroy humanity is her second step. And the third step, was staying in men's world to continue steve's fight to save humanity from itself