I am honestly torn between the butcher of Ariqa and the Bantallian decimation. On one side on Ariqa she eradicated life on the planet surface which is just mind boggling.
On Bantallia though there is the much more visceral human element, as this is a tragic situation where the situation had no need to escalate so drastically and could have probably been resolved with only a minimal drop in efficiency, possibly even lesser than the one that happened due to the loss of life and material damage, if it wasn't under the rule of a fascistic empire that considers the showing of any weakness as catastrophic.
But in the end I think I vote for Ariqa, as on Bantallia there was no way that Vaere could have or would have wanted to manage the situation in a way acceptable to her superiors except by ignoring it and passing it to some other would-be butcher.
On Ariqa though, she was already and admiral, yes, a newly minted one, whose position was very fragile. But as you climb up the ladder, so increases your action range. And here I think she shows her true colours, as she is among the first to end the situation, but her solution to do so is to render a planet barren and kill all on it. Here Vaere shows that brutality is not something she dabbles in, but something to which she resorts as soon as the pressure mounts up.
So... after making a mammoth of a post of which I am slightly embarrassed myself, I vote:
[x] -the butcher of Ariqa.