Yes, slave dude is hella sus. Sus. He screams red flags. BUT, here’s my theory: I THINK...

Yes, slave dude is hella sus. Sus. He screams red flags. BUT, here’s my theory:
I THINK HE MIGHT JUST BE A RED HERRING. He IS suspicious, but there are some things that just don’t add up.

If I remember correctly, the slave is one of the first people Clea meets, and she interacted with him before the banquet with the Duke when she is reincarnated. This is important to note.
When Clea enters the banquet with the Duke, we have the famous line: “How is she not dead yet?”
Clea says that this is the presumed killer, so we’re going off that. The line itself implies the killer did not know Clea was alive, despite Clea appearing in high society twice after she reincarnated (that we know of).
1. When she asked the Duke to marry her in front of everyone but got rejected.
2. When she asked the Duke to talk about a marriage contract over some wine.

But, the slave does know Clea is alive. So that line simply makes no sense. Even if the slave was working for the real killer, than he would’ve reported it, but we still get that line.
With this, we can rule out the slave, he’s suspicious, but it just DOESN’T make sense.

I think the killer might be...Marquis Salami.
The perpetrator must not be that active in high society, since it took Clea appearing a MINIMUM of 3 times at social gatherings for the killer to know that she is not dead. Plus: Clea marrying the Duke would’ve been HUGE NEWS. Huge news that everyone would be talking about: yet the killer STILL didn’t know that Clea was alive? Well...
Marquis Salami is described as a mysterious character who rarely appears in high society, and keeps to himself. It would make sense for him not to know about Clea being alive, since he is pretty inactive when it comes to social circles.
Not only that, but he SUDDENLY invites Clea over using an invitation. Judging by the reaction of the maid, this isn’t common. Why would he send an invitation to...Clea? She’s really unpopular, described as a demon. The big problem here is: people don’t mask their hatred for Clea, going as far as to send hate mail.
If Clea DID know Marquis Salami on good terms, but can’t remember, people would have pretended to be nice to her, since Marquis Salami is very popular despite not showing up often. If Clea WAS friends with that Marquis Salami, a lot of people would want to get on her good side. They’d want to get close to Marquis Salami.
The spontaneous invite just doesn’t make sense.

But of course, you’re wondering: “But what about the line? ‘How is she not dead yet?’”
Of course, Marquis Salami was NOT at that banquet, otherwise there would’ve been a HUGE scene. A far-fetched claim is that it WAS Marquis Salami, he may disguised himself, as mentioned, he DOES need to do that due to how good-looking he is, and magic does exist, so there could be some form of illusion magic that can disguise his face.
But, a more believable claim is that it was a subordinate of Marquis Salami, or someone conspiring with him to kill Clea. Marquis Salami might send someone to social gatherings to keep him in touch with everything. So he doesn’t fall behind.

Clea was a Countess (during the murder) and was murdered in her own home, so, it makes sense. Marquis Salami definitely would have enough status and money to be able to cover-up a murder.

So, that’s my theory, Marquis Salami is the killer, the slave is a red herring, and is purposefully trying the throw us off.