Chapter Text
The entire trial is a big joke. It's the first time we get to directly see the government in action, and it's the official introduction to the Scouts and the Police. It is gloriously ugly and demonstrates the disconnect the upper class/inner city has with the general populace. It's pretty obvious that they don't really give a shit and just want to maintain the status quo because it favors them. The military police, described as maintainers of order, assume a stance that would perpetuate it by eliminating Eren immediately. It doesn't change their situation in any way, but removing Eren keeps their problems out of sight, out of mind.
The scouts, by the nature of their occupation, end up taking the opposite view. To them, Eren isn't a threat, but the opportunity of a lifetime, and they fight tooth and nail for him during the trial. Erwin states they were fortunate to have been able to play their cards at the most opportune moments, suggesting he was the one who orchestrated the entire thing. Assuming this to be the case, Erwin basically played them all for fools. He preyed on their deepest insecurities and framed the Scouting Legion's goals to be exactly what they all wanted.
Before he could actually do that though, he whips them all into a state of emotional hysteria. Once they're all good and emotionally compromised, he rephrases his original proposition into exactly what they want to hear: that he'll take Eren, this monster who disrupted their peace, away from them, outside the walls where he'll likely die. It's exactly what Erwin wants, and he manipulates them into thinking that what they want is what he wants. (Sidenote: this is when Erwin instantly won himself a place as one of my favorites.)
In other words, all the scouts are acting under Erwin's directive to make sure this happens. It's an act and everything they do during the scene is calculated to bring about a certain reaction, meaning their explicit actions here shouldn't be considered direct reflections on their actual personalities. In the anime, Romi Park really hams it up as Hanji while escorting Eren to the courtroom, and the over-exaggeration I think helps emphasize that it is an act. She sort of drops it a little when she says Eren is probably better off not knowing what to expect and they'd simply have to trust him blindly.
Going by this, it'd be reasonable to assume that everything with Levi during the trial was an act too, a calculated exaggeration to shove the ugliness of the inner city's fears into their faces. Their position behind Wall Sina shelters them from the reality outside the walls, and Levi's display is unrelentingly and unapologetically brutal and violent. It was incredibly shocking and uncomfortable for them to watch, which is precisely why it was necessary.
It was timed really well too, because the moment everyone decides that Eren is better off dead, Levi jumps in and proves to them that yes, Eren can be controlled. His language, which is degrading and dehumanizing towards Eren, appeals to the conclusion the military police and inner city have already drawn of Eren. The way the trial proceeded so smoothly (Erwin's plans anyways; otherwise it's a giant clusterfuck) casts an even greater degree of insincerity on Levi's actions. This is supported later when Levi asks Eren if he hates him, making sure that Eren understands that it was necessary and Levi didn't just kick him in the face for shits and giggles.
However, we learn later about Levi's criminal background, and we see that scene he has with Annie while her titan form is restrained. From what he says, it's implied that Annie has been actively sabotaging the Scouting Legion's efforts for some time and was personally responsible of the deaths of many of Levi's subordinates. As Levi has been shown to care deeply for his subordinates, Annie's actions have provoked him and provoked him strongly.
He's angry at her for good reason ... but the way his anger manifests is really disturbing.
He threatens to cut off her limbs, her human limbs, and casually ponders whether or not the limbs will grow back. He reassures her that he won't kill her because they need her alive. During the whole scene, he is psychologically tormenting and threatening severe physical harm on her person, and he describes the experience as fun for him. He's reveling in her pain and distress and that is something really scary.
What makes it worse is that he says he sees a connection between himself and Annie, who has been responsible for some of the most brutal deaths in the series so far. She stomps a person flat, grabs someone and scrapes his body in half with a tree, cuts out the nape of someone's neck, and it's implied this isn't the first time she's done stuff like this because the Scouts are aware of her existence. Levi KNOWS very well what Annie has done and is capable of doing, yet he still draws the comparison. It might as well be a confession on his part: he does enjoy inflicting pain on others.
However, this is under extenuating circumstances, and Annie has done a lot of draw Levi's ire. The thing is though, Levi has a criminal background. It's very likely he had some unpleasant encounters with the military police before he entered the military, and assuming he spent his pre-military days in poorer areas, he probably doesn't think highly of the inner city dwellers. The upper class has likely done just as much to leave a negative impression in Levi's mind.
THAT'S what makes Levi's actions during the trial uncomfortable to me. There's a similarity between his actions with Annie and his actions at the trial that adds a degree of sincerity to his role at the trial that previously hadn't been there. He's angry at Annie, angry at these institutions of power, and his anger manifests through violence. It's not exactly what I'd call healthy.
It implies that he enjoyed what he did at the trial, which was more than just beating Eren half to death. He got to defy their authority because he's simply too valuable, and he forced them to have a taste of the reality he lives and they deny. He got off not on the physical act of beating Eren, but on the distress he inflicted on the people he despises: the military police, the merchants (especially personal since his background implies he lived in poverty for a while). While his remarks (calling people pigs) were intentionally crafted to be inflammatory, they still had to make reasonable sense coming from his mouth.
The distinction is important because Levi treats people he likes/respects reasonably well, if not respectfully, and I think he does like Eren and see himself in Eren. Had he not, I don't think he would've vouched for him. I think Eren's desire to eradicate the titans was pivotal in swaying Levi's opinion. It just really made it clear that they shared this disgust and hatred of the status quo and wanted to tear it down. They're both angry people who express their anger through violence. Eren defaults distressingly quickly to violent solutions whenever confronted with something that doesn't suit his rigid sense of morality. Violence is coded into Levi's identifying title as Humanity's Strongest Soldier.
tl;dr Levi demonstrates sadistic behavior during the series, but it isn't because he is a sadist. He is usually provoked into anger, and expresses his anger unhealthily through inflicting pain on the aggressor and reveling in that. Please stop turning him into an abusively sexy boyfriend for Eren because he's not interested in beating Eren because Eren has done absolutely nothing to piss him off to the degree that Annie and the inner city do.