My poor attempt at cracking the case by gathering hints from what's shown to us
Let's unravel this mystery by topics.
So we just recently learned that Holt doesn't exist, or he's merely an illusion cast by a magic user. Still, when Stephen first meets him, his assaulters do call his by his name.
Which could be explained by how Holt's facade is able to alter people's memories. We do know the culprit is skilled enough to mess with Loki's memories, an asgardian god, let's not forget that. In short, this could mean nothing, or it could mean that the impostor is also able to alter Stephen's and regular asgardians' memories.
But what about Holt's family? Are they all illusions? Are they all fabricated memories?
In addition, his story is compelling, if not very common. A person who doesn't want to continue the family's business, even though he's still frustrated.
Let's also remember that he was the one who talked to Liva about renting a room for Stephen and is constantly interacting with extras.
Another important thing that I've noticed is that Holt is the bridge between Stephen and Asgard. Our good doctor trusted him well enough to reveal he's involved in a murder and investigate the ax.
Although Holt's resistance to be part of things, he doesn't turn his back on Stephen.
And then this panel caught my attention.
"....my apparent jocularity is merely a mask I wear so as to not draw attention."
Couldn't we consider that both Holt and Aslak are gregarious? Because they do know everyone in Asgard, as Roskva stated when recommending her own boyfriend to show Stephen around. There's this feeling that these two characters are overlapping, somehow, which will be even more evident when Aslak actually confesses that the Ax of Esquisite Decaptations was in his possession. And that leads us to our next topic.
Next, we meet Aslak, the one sent by Hulda. There's something about him that doesn't sit right with me. I'm all down for soft men and how they love their wives (Stephen is precisely one of them), buuuut it's about how he's perceived and constantly underestimated by his peers and himself.
Whether it's naivety or just a facade, it's very hard to tell. Would someone be naive enough to trust an outsider who was last seen with the victim just minutes before her own death?
Moreover, Aslak is quite aware that Hulda treated him badly, calling him an idiot and being rude, despite his admiration for her.
This panel, specifically, has this line. "Snowing. It's always snowing is Asgard." I'll come back to that because it does sound suspicious and contradicts with another detail in issue #4. Let's leave it for now.
We just recently learned that Loki's memories of the first murder were altered. So here's how we see it at issue #1.
That's Stephen's POV. In issue #4, Loki reveals they have slightly different recollections, leading them to discover that Loki was actually teleported during the very moment of the crime while the impostor disguised themselves as Loki in order to frame them.
The second attempt of murder was at Stephen's room, and the only clue left was the Ax. We need to consider that Aslak was the last person to see Stephen before the assault.
The third attempt and second successful murder happened during the dark troll hunt. It wasn't directed at the usual victims, per see, but Stephen does raise suspicions that someone doesn't want him to succeed.
And lastly, someone impersonating Roskva (because I hardly believe it's the real warrior) is after Stephen. WITH THE VERY SAME AX.
Also, regarding the first murder, we don't know much about the dagger, but we do know it happened on summer. And that's the precise moment that Aslak's line, "it's always snowing in Asgard" doesn't make sense to me. If there was no fire (summer), then it wouldn't be cold (snowing). See why it feels off? Bear with me, there's reasoning where I'm going.
So far, we've known that the culprit has the ability to alter memories of powerful entities and nullify Stephen's magic, which isn't an easy feat despite his present circumstances.
Besides, there's no point in trying to guess the culprit by their appearance because they know how to shapeshift and/or manipulate people's memories.
Those details imply that the culprit is actually very skilled at sorcery. What characters have we encountered so far who are magic wielders? Hulda, who's currently dead, and Aslak, the one called eternal apprentice. There's also Loki, labelled as innocent by the narrative, and possibly the magic shop's owner (although not established, one can sell magic stuff without practicing it).
With that being said, from now on, this is all speculation from my part and take it with a grain of salt because I have my prime suspect for a while now, but I swear I have reasons to believe so. It's very hard for me to ignore Aslak's presence everywhere.
He's always mocked and labelled as weak and incompetent, yet he remains kind. Which is hard to take in Asgardian culture. Hell, we have Loki themselves to prove our point, being constantly bullied for being smaller and weaker than your average Asgardian. Also I don't believe for a single moment that Stephen often being called tiny man/doctor is a coincidence. It's a cultural thing if you don't fit in and fall into the expected archetype.
Moreover, the culprit HAS to be amongst the ones who joined Stephen on the dark troll hunt. "Oh, but he was seen in this panel."
"But what would be his motivations?"
Now let's take a deeper dive. And yes, it could be a stretch. But first of all, Aslak impressed Loki due to his abilities. The guy called eternal apprentice while the whole book is trying to convince us he's kinda clumsy and not very good at it. Secondly, he got the ax. I don't know about you people but I'm not convinced regarding his excuse. When we add these facts with the idea that he's actually a very skilled sorcerer and no one really pays attention to him (Loki being the first AND getting framed), all I can think of is that he wants recognition and power. Maybe become the strongest sorcerer in Asgard? Maybe a skald or Sorcerer Supreme?
To do that, he needed Loki gone forever. Because every Asgardian knows how old of a tale is the relationship between the god of thunder and the god of stories. They come and go back, which means the banishment wouldn't last forever. But when you add more wood to the fire like, say, a murder of a very important person like Hulda? Thor would have more than enough to cut ties with his sibling for good.
Then enter Stephen. He wants the role of Sorcerer Supreme and he will succeed since he's that good. Approach him, befriend him and use this bond against him. Both Holt and Aslak filled that role of becoming the bridge between Stephen and Asgard, both are very sociable, both know lots of people.
That's why the murderer tried to sabotage their mission. Stephen becoming the Sorcerer Supreme of Asgard is bad news to the culprit. And I can't think of any other character in this book who would want to prevent that from happening.
Lastly, what about Roskva? Why would the culprit impersonate her? I actually DO have an answer for that. And the answer is Gorm! "Who?" The guy who's always around her? Always exchanging fists and fighting in a friendly manner? The one who accidentally put a knife in her shoulder and feels bad about it? He's everything Aslak is not.
In a society where strength and virility are the most desired virtues, Aslak has none of what it's considered attractive, even the ability of growing a beard to look like a "true" sorcerer.
Out of jealousy and bitterness, he could frame Roskva, even though she never gave any reasons for him to do so. We call that male insecurity.
Aslak appears to be the only one who has plenty of reason to connect all these people and frame them accordingly. Besides, I'm pretty sure Stephen already figured it out and sent him on a trap.
"Whether he (Thor) believes you or not is immaterial."
He is not seeking Thor's judgment, he's seeking Thor's presence. The moment Thor learns Stephen's whereabouts, he's kicking the door and arresting him. And that's exactly where Thor needs to be: the moment the culprit is about to kill, catching him red-handed.
In my head, all these clues and hints come together and point to Aslak. And if it's not him? Well, then this post was entertaining while it lasted 😅. In any case, he's got the motive, the means and the power to do so. I can't really explain Holt and Aslak as of yet, all I have are theories (maybe they're the same person? Maybe Aslak used the illusion of Holt's figure to look less suspicious? Idk).
All we can do now is wait for the last issue. I rest my case, your honor.