The best way to explain this to her would be the simplest and most direct route, even if it ran the risk of sounding a little insensitive towards the hurt sentiments that caused all this. "I killed Vincent Abrams, or at least I thought I did, not so sure anymore. He was a friend of the Alliance but started behaving erratically and getting close to the Technocracy, of course that did nothing to mitigate his erratic behaviour. It got bad enough that the Order had reason to believe he was compromised. So a decision had to be made, and I made it. His life for the LFR."
It struck him that things felt so much simpler back then, at least in hindsight and this would cause his expression to grow distant before he caught himself reminiscing and continued talking. "My mistake was doing this in secret. I didn't tell anyone else in the Alliance who cared about him, so you can imagine they were upset to have this hidden from them. Didn't end with this though, they seem to hate every recent decision I've made and feel it's all progressively perverting the cause, almost as if the cause only belongs to them. They hate that we're dealing in drugs, they hate Knight but I can sort of see why, and they also hate you. That I wasn't willing to let slide. They did the exact same thing the people we're truly against do. Belittle what you've gone through to get here, and disregard your own personal struggles. It must be nice thinking with such narrow focus and not understanding how Nations are actually built. It takes all sorts to make a Nation. Nothing I could say was going to make them see reason. So they lashed out when they felt provoked and ran when it seemed like the job was done." Strangely, he didn't seem angry or offended by any of this. He realised that on many levels this conflict was unnecessary and it served neither of their interests, only the interests of enemies.
"They've all been disavowed now, there's no going back. Just like I might not be able to fix their grievances with me, there's no way to undo the perception everyone else has developed towards what they did. Nobody gains anything from this except for our usual enemies who might have been informed about what happened. So it's only fair that massacring them receives a more enthusiastic priority. Can't afford to look weak, can't afford to be weak" Finally he felt he'd mentioned everything there was and even addressed the course of action looking forward. But he wasn't any less uncomfortable than he was before saying all this, and in many ways it just made him feel worse and that was written all over his face.