Anime and manga in general is pretty well filled with powerful villains turned either good or morally ambiguous rivals. Toriyama's DRAGON BALL did this on several occasions. First with Piccolo, Vegeta, Android 18, and Buu. That can even be extended to various other franchises. That's a difficult premise to pull off and keep the character's motivations. Vegeta just stands out to us, because even though he came to the side of the Z-Fighters, he never lost his rivalry with Goku.
As I began watching Trigger's KILL LA KILL series in early October, I went in with a single view of Satsuki Kiryuin. She's dominating, head strong, ruthless, and thought little of humanity that doesn't conform to her ways. That seems to be the impression we have of her for the first few episodes, then Episode 3 "Junketsu" comes along. Slowly cracks in my preconceptions started to develop. Back when Satsuki challenged the friendship between Ryuko and Mako, she walked away smiling as she seemingly failed to break the pair apart and prove the point that humanity are "pigs in human clothing" that are blinded by greed and need taming by force.
I've read some complaints that several of the story flashbacks didn't add much to developing the members of the Elite Four, but was that even their purpose? I actually thought that we learned more about Satsuki. Just through the eyes of others. From her position, she saw people from privilege exploiting others or depending on their parent's influence to avoid the consequences of being a delinquent. Is she wrong that greed corrupts humanity?
When Ryuko lost control of herself as she fought against Nui. Satsuki tried to get through to her with words before she attacked. Even when she was ready to put Ryuko down, she held Nui off to allow Mako to save her. Several days later, she was straight forward to Ryuko and admitted that it was on her orders that Nui went after her father.
With Senketsu's destruction, I'm once again left confused. Satsuki has rescued Ryuko again, but she also took away the scraps of Senketsu. Although, the closing shot shows a single shred is left in her hand, and Aikuro found her carefully covered. As I was thinking back, I started to wonder if that piece was always in Ryuko's hand.
I went back and watched the episode once more. In all the scenes showing a vulnerable Ryuko, nothing is seen in her right hand until Aikuro finds her. Only then is the piece rather obvious. This is a series that pays attention to the little details. Just look at the episode with the PULP FICTION cameo.
It could be very likely that the piece was always in her hand, and it just wasn't illustrated. Although, if Satsuki is the sort of person who would cover an enemy, isn't it just as likely that she could have left that piece with her on purpose? I'm not making the claim that she's doing this as some sort of fondness for Ryuko, but she seems to want to keep her around. Likely for some purpose of her own.
Other recent events have also suggested that Satsuki may not be the good little soldier of her mother's organization that earlier episodes had suggested. Even Vegeta played along with Frieza until he had the chance to turn. The biggest difference, Satsuki is far more competent. Think about this. For as much as Ryuko has been fighting, in the end it's only helped to make Satsuki stronger.