How Livio and Razlo test the audience
(Psst! You should probably read the preamble to this first. It gives some background into how the audience should be interacting with the manga at this stage of the narrative)
It's somewhat overshadowed by the... various other things going on in volumes, 8, 9 and 10, but in terms of the ethical dilemma being debated throughout the text, things are actually looking rather good - Wolfwood has been the biggest argument against Vash's pacifism (side note: take a shot every time I use something resembling those last two words in this essay) and now not only has he proven himself a loyal ally, but is going out and sparing people's lives himself, it's easy to see how the narrative is ramping up to 'potential redemption of Millions Knives'.
But that's still a pretty big jump, isn't it? We're going right from 'guy who is actually pretty nice, actually and just in a bad situation' to 'mass-murdering genocidal extremist whose only potential option for redeeming is the sad backstory that radicalized him in the first place'. Maybe you could suspension of disbelief it, but ideally we need something in the middle. A stepping stone, to push Vash's ideology even further.
(Yes there are much clearer panels of Razlo. But they're not the best like this one is.)
I've gone over Livio's deal before, but to sum it up - this is a man who does not care to be someone anymore. He has his few loyalties, gives his body to them, and probably spends most of that time as nothing more then a husk.
This is probably in my top ten favorite panels because of how clearly it communicates Livio's character without Livio himself saying a word. It's your 'evil bad guy glare', but it's so so pained and yet so so hollow at the same time. This is what chronic depression looks like.
Razlo on the other hand is introduced to us via the 'evil alter ego' trope, which I'm typically not very fond of, but I think for him it works. Specifically because it works with the audience's expectations in a way that is so very Trigun - a story that has told us, over and over, that people don't just do evil for shits and giggles. At this stage in the narrative, our expectation as an audience - the way we've been positioned, should be that there is something more to this.
For one, there are very few things that can match the utter mic drop moment that I had reading this for the first time and going from 'oh goddamit not the evil alter ego trope again' to 'holy fucking shit the Eye of Michael groomed a child, like probably only 13-or-so child, specifically by taking his own disability that he barely even knew existed at the time and used it against him just so they could use him as their personal murder servant.
Think of them, then as a sort of 'practbice run' for Knives. Deeper in the murder cult hole than Wolfwood, but not in control like Knives - still under the controlling hand of Chapel, and once you get rid of Chapel, their attitudes start to shift. The fact that it's Wolfwood specifically appealing to Livio is also significant to mark this as a stepping stone - it's Vash's ideals becoming power for others, a 'passing of the torch' if you will.
Look, they even mirror the twins through personality! You have a more distant and reserved side of the duo, a more gentle (but by no means not dangerous!) person and known crybaby.
Then you have the angry side, a gaping wound of an unhealed inner child, over-protective, paranoid and emotionally-driven to a fault.
But then there's the question of designs. After all, if Livio mirrors Vash and Razlo mirrors Knives, then each should associated with the same side of the face as the twins with their moles - and yet the visual shorthand used to show when Livio is fronting is focus on his left eye - the same place Knives has his mole.
But then of course there's the fact that if you look at the panel with Knives above, the focus is on his right eye - so is it one or the other!?
The blurring of roles here is a bit more obvious with Livio specifically. Tell me you haven't looked at a panel like this and gone 'hey he kinda looks a bit like Knives with that hair'
That of course leaves the question: What exactly does this shake-up tell us? Is there meaning here or am I reading too much into it?
If I had to take a crack at it, I'd say that the uncertainty here is the point. It reminds us that we don't really know either of these two that well, or that they're not a threat. We don't even have both of their word that they're on the side of the good guys now - only Livio's -, and it's a promise coming right after he and Razlo killed Wolfwood. So perhaps it's not surprising that Vash might be getting echoes of his brother here. It's a set of coding that feels strange and contradictory in its motifs - yes these two mirror the twins, but the analogy isn't clear cut, but that's also kinda what Trigun is all about - the nuances of kindness hiding in sharpness and strength in even the less outspoken.
The fact remains that these two cannot exist without each other, intertwined as the embodiment of the complex human nature - often at odds, but when it manages to come together, it comes together in arguably the coolest fight in the whole damn text.
Forget Legato or even anything to do with Vash, because I'd argue that this is the precise point in the narrative that thematically sets in stone how the fight with Vash and Knives is going to go down. It is the point that the story outright states that unity of the self, with others, is a thing, and it will succeed.