Stussy being an antagonist fits flawlessly into the picture. It fits even better than if we were to categorize her as an ally.
In chapter 1072 she went against Lucci and Kaku, because look what Lucci did: He ignored the explicit rules for their mission: Do not destroy any of the lab’s precious interior.
Instead to adhere to that rule, Lucci ordered the Seraphim to destroy the Labo-Stratum entirely. This is not what I understand as “being careful with the lab”. And Lucci also went against the explicit command of Akainu, who ordered Lucci (CP0) to not “under any circumstances” engage the SHP.
Well, Stussy made them stop in her way – after she asked Lucci more than once the following:
Chapter 1070 Lucci ignored her:
And again in chapter 1071 Lucci did not want to think about stopping:
In that way, she obeyed Akainu’s command to the letter and could also infiltrate the SHP/Vegapunk side. This is also the reason why she didn’t help Kaku and Lucci – because of the command from Akainu to not engage the SHP in battle.
We also need to remember that she is the product not only of Vegapunk, but of MADS. That means that Ceasar, Queen and especially Judge could have had a hand in creating her. And look at how Ichiji, Niji and Yonji turned out: They do not feel anything, which makes them psychopaths. Now look at Stussy and how she behaves when she fired that Shigan at DuField. And than look at her face as she tricks Lucci and Kaku. She is smiling both times, while she inflicts pain upon others. This is telling. Because this is a similar behaviour we see with Sanji’s brothers.
I dare not to say it, but Stussy behaves like them.