I absolutely agree. Negan suffers from a lack of context; unlike Rick, we didn't get to see the appalling things which created him, so we've a harder time excusing his scruples. What Rick did with the outpost to precipitate Abraham's death was a dick move, and from the Savior's perspective, I think Negan's reaction of killing 1-for-30-odd was pretty equitable. And Glenn... Well, Daryl, geez.
I like it too! The more I whittled away at Six's history, the more I figured she's going to be pretty much perfectly okay with Negan's methods. The Sanctuary works, and it works well, and the communities under their 'protection' need the Saviors' stability or else they would've been able to resist them. So the Courier will probably be bought and paid for by Negan's reasoning. But she does meddle. And if she starts thinking maybe Negan's gone a little too extreme to be sustainable, and Alexandria/Hilltop/etc. are starting to get Ideas, she'll absolutely try to act.
But she'll probably try to do it off-the-record, on the QT, and strictly hush-hush. The Courier likes mucking about in the shadows, so she'll definitely be biding her time, figuring out who in the Saviors is on the fence when it comes to loyalty, that kind of thing. In the mean-time, though, Courier will do her best to stay on Negan's good side. And if she does get this sense that folks are getting restless, and they're about to try something extraordinarily dumb, I imagine her first move will be to try and talk to him. 'Cause why kill the king if you can just steer his hand?