The last line of the chapter is really important here: only a speck can understand. Eren like most specks is helpless to do anything about the injustices faced by his people, so in most occasions they don't lash out or seek revenge since it's a hopeless endeavor. Instead they keep all those emotions under the surface for their own safety. However Eren when around his own people is a bit more forthright with violence (hence his fight with Oscar which started over a rude comment, and history of fights according to Gabriel) so he's not indifferent when it comes to people who he can feasibly interact with. With Maddie since she was a normal person who he could not feasibly do anything towards, and she had also earned his admiration, he responded with indifference not because he is opposed to vengeance but rather there was nothing he could have done to her if he wanted.

And this is where we get to Jisoo. She is a speck who became human and also part of a very wealthy, influential family. So she is in the very unique situation where she actually can get revenge on people who have wronged her. Unlike Eren and pretty much every other speck. He stays with her for a couple reasons. The biggest is what she did was pretty much emotional manipulation by bringing up his parents, and asking if he would do anything differently if granted the opportunity. As a speck he is one of the few people who can understand why she's so violent and vengeful towards bigger people. Furthermore, he feels indebted to her since she not only saved his life but also has provided a wonderful new life for him. Subconsciously he knows that if he left her that could all come to an end, and his old miserable life would come back. I hope this explains the reason for why he for the time being chose to stay with her.

As for a spin-off, I'm flattered but this story is so far from finished I couldn't really consider one. Grace is definitely a character who might get her own fun spin off but for now it would be best to keep them all contained to this story.

I appreciate the thoughtful review!

So as for their justification with Thomas, in the previous chapter it was stated that one of the reasons Jisoo outright killed Thomas was to prevent him from sharing his side of the story, since she wanted to sculpt what Eren thought of her actions. She knew she could manipulate Eren with memories of the trauma from his parents murder, and he would be more receptive since Eren already feels so indebted to her as is. Moreover it is worth noting that the world that Jisoo and Grace want may not necessarily be a place where people like Thomas exist to begin with, both have shown themselves to kill people for even the slightest provocation. To them Thomas is not really a person to be reformed and more of a thing to be used and then discarded, and they will simply use whatever justification they need to do Eren will remain on their side. 

As another reviewer noted for this chapter, it could be interpreted that Jisoo either lied or omitted certain parts of the truth. Remember that her entire goal with this conversation was to make sure that Eren remained at her side, so she would be incentivized to paint her actions in a positive light and downplay Thomas’s place as a protector to his sister. If she had simply admitted that she killed Thomas because she felt like it and she enjoys how it makes her feel, Eren’s reaction would have been very different, so she came up with an excuse closer to home he would agree with. 

Bear in mind that Jisoo considers herself to be a ‘monster’, as she agreed with Alice when she called her such. She doesn’t really consider herself to be a ‘good person’. To her being a good person means doing what she considers furthering her family’s interpretation as the greater good, not anybody else’s. Furthermore she is a former speck who suffered for years as such, and makes choices based on her emotions before anything else. I’m sorry that you haven’t enjoyed these past few chapters but feel like it’s important to understand the characters motivations and perspective, since it drives their decisions and what they say to each other may lies or half-truths, which a lot of this chapter was.