I'm really not sure what point that moment served. Was she deciding that violence isn't always necessarry to stop her enemies? Having a random moment of empathy? Didn't want to kill a woman? I don't know.
Ares rips Dr Poison from the car she was escaping in and presents her in front of Diana to kill while Diana's in rage mode, hoping to have her be on his side, and I guess her killing Dr Poison mercilessly would be basically sealing the contract. It's the lofty "Return of the Jedi" scenario where Luke giving in to kill the emperor would be giving in to the dark side.
So her not killing Dr Poison was her making the point that humankind can be terrible like Ares says, but they are also "so much more".
Even though it's the meaning behind what killing the person means for the character in a specific context, I always found this scenario in film to be kinda weird since Diana (and Luke) killed on their way to getting to this ultimate choice moment.
I hate that theme independent of its use in the film, but having this electric guitar riff (or cello? Dunno.) come out of nowhere in what had been to that point a pretty traditional, if excellent, period score took me straight out of those action scenes. It's the worst scoring decision I've seen in a film since Ladyhawke.
I'm mixed on the wailing cello theme. The song seems to alternate between two main melodies, and I love the second one.