
My best guess is that she is the empress. It would be easier the kill someone of a lower or equal rank and get away with it, but trying to murder the empress is far more risky. Even if he did try to blame someone else, the other nobles know what he is capable of and his history with the empress, he would still be the number one suspect. Even, if it looks like a natural death, he would be investigated because as I said they know what he is capable of, mostly. That's just my guess though.

That's a good guess, but it doesn't make sense when you remember that he killed his father, the actual emperor, just to make his brother rise to the throne.
Thus, technically the brother is of a higher standing than the empress, considering that he is the emperor. While putting the blame on someone else would be risky and rise suspicions from the other nobles, if she died of natural causes, who would have a high enough standing to question the emperor himself?
My best bet thus far would be sentimentality, but Arsen doesn't seem like the type to feel this way, considering, again, that he killed his father and very obviously hates the empress.

The family that backs the empress dowager. It is easy to kill an emperor with less prominent allies but it's hard to kill the empress dowager if people behind her are old power but the way how he killed the emperor was kinda sus tho. Did he really kill the emperor especially he didn't participate in becoming emperor and sided with his brother? Because, we are seeing this in MC's perspective. He doesn't have any MO to kill the emperor nor is he the one who just suddenly kills the emperor. Idk, the way it's set, it seems kinda sus

They are already killing the families who back her up one by one, so the logical conclusion is for her to get killed as well. Thus, as I said, it's easy to make it look like a natural death or an accident, I am sure they could make it if they wanted to. There must be something else.
As for him killing the emperor, MC says that his is written in the novel she's read, so I am assuming that this is the truth.

idk, because as far as I have read all the manhwa about that bit, the novel doesn't really go into the nitty gritty stuff behind why they killed someone important or they were framed as someone who killed that someone important. If they really are killing them one by one, the most logical thing is, they're killing noble families that are henchmen, not the ones who are in higher position because the empress wouldn't still be in that position if someone powerful isn't backing her up. You can't just sit or have a title while not having any power in your pocket. That's like telling other factions that you're a free kill. And again, it's really suspicious that the duke who obviously stepped down from contesting the throne from his brother, to help that said brother kill the emperor, with no other reason because the book said so. That's fishy and he doesn't even a strong motive to kill the emperor whether he was just a puppet or has his own backing. It doesn't benefit him or his faction but more suspicions towards them. Then if he just killed the emperor with no viable reason but just to make him look more evil, that's just a big plot hole right there.
So, he can kill other nobles... Why can't he just kill the empress as well? Like, she's clearly the biggest threat for them, it would certainly be easy to just kill her and frame another person they dislike? Sounds like that would be a way better use of their efforts than trying to find evidence on her.
"But her family would be a problem" - that's where the framing comes in place? Blame someone else? Or even find a way to kill her that resembles natural death?