The fact that Kaidan has the opportunity to shoot Brooks after you reclaim the Normandy in the Citadel DLC is so interesting to me, and I think for Emrys it's actually a really huge deal for them. (Obviously, from a gameplay perspective, all squadmates can shoot her, so of course Kaidan would be included, etc. etc..)
There's a point way back in Saren's base where it really hits home for Emrys that Kaidan has become a significant part of his moral compass--even before they're anything more than friends. When they come across the docile indoctrinated salarians, Emrys opts to execute them out of mercy. It seems a more honourable death to be looked in the eyes by the person pulling the trigger than being eviscerated in a jail cell by a bomb they didn't even know was coming. There's not really any chance for them, not really a life left to live. Perhaps if the circumstances were different, but they're not. It's not strictly a necessary action though, so he bloodies his own hands--doesn't ask of his squad to do the same. Regardless, Kaidan makes it clear that he's not a fan of Shepard's decision to kill them.
Not long after, they encounter Rana Thanoptis--and under any other circumstance, Emrys would have shot her without second thought. He has a few reasons--vague sentiments of preventing her from causing future harm, knowing what corporate scientists are like--but if he's honest, they're flimsy justifications. Yet, that isn't what first makes him hesitate: it's the feeling of being on thin ice with Kaidan. Having just killed the salarians, killing Rana feels like a decision that would genuinely harm Kaidan's perception of him--especially without a damn good justification.
It feels frivolous on the surface, to spare someone because the guy he's flirting with would look at him differently, but it's more than that. He's a better man with Kaidan, instinctively slows down his decisions and truly rationalises them, purely for having Kaidan's morals as an anchor, as a point of reference to judge his own against.
And then, three years later, Kaidan shoots Brooks.
And he didn't have to. Not really. Realistically, she wasn't going to get very far anyway. It's justifiable from a purely pragmatic perspective, sure, but not particularly ironclad. A sort of grey area that ME1 Kaidan would have taken the more compassionate side for--or at least aimed for her leg, perhaps. But here, he just pulls the trigger and watches her drop. And for once, it's not Emrys moving more towards Kaidan's morals, it's Kaidan moving towards Emrys'.
Which isn't to say Kaidan becomes more 'renegade', more ruthless, because that isn't what this is. They're not one good and one bad. Instead, Kaidan's comfortable in allowing himself to set lines when it comes to protecting the people he loves that are no longer last resorts. He's allowing himself to have more control over a situation by pulling the point of action away from 'necessary' into something more grey, something where things could still go either way. He's opting to make the hard decisions before the action becomes obvious and inevitable, and even without the pressure of needing imminent action, when the conditions are right to him, he will make the choice to kill. But more personally, he's showing that his love for Shepard has just as much influence over how he sits with his morals, as Emrys' love for Kaidan has over his. Emrys never doubted that, Kaidan never makes him feel anything less than incredibly important to him, but there's something about seeing it in action knowing it didn't come from a conversation about integrity and hard decisions, but simply from loving someone so deeply. And, hell, maybe it's also the pride in seeing Kaidan do something unapologetically brash that he would've done. It's damn hot to watch, if nothing else.
