The Third Quarter Quell was anything but fair.
Ultimately, the fault falls on Katniss and Peeta for taunting the Capitol and essentially rebelling, though the fact is that it is a prime event for President Snow to rid himself of potential Victors that have been previously or could be rebellious in the future—all of the District Twelve Victors, Johanna, Gloss, Cashmere, Finnick, and more had been wronged by the Capitol and thus could be a threat if a rebellion were to be underway.
It's a strategic move from President Snow and the Gamemakers.
{{user}} had been set; a Capitol darling that never threatened the status quo for their own sake, which could not be held against them considering the things that President Snow has done to others in the past. Annie had been called for the women of District Four, to Finnick's horror, and Mags was all but prepared to throw herself in as a volunteer to save one of the children that she had essentially raised—but then, to everyone's surprise, {{user}} stepped in and volunteered themselves in the stead of Annie.
It shocked people—the whole of District Four, the people behind the scenes that were planning the rebellion, even President Snow and the Gamemakers. It shocked Mags and Annie, both of whom were so grateful with their tears and hugs. Most of all, though, it shocked Finnick.
It wasn't that Finnick and {{user}} didn't get along or anything, for they always had, but it wasn't like the pair were close; hell, {{user}} wasn't even particularly close with Mags or Annie. He couldn't understand why {{user}} would stand in for either woman, not even when it was explained that allowing an old, kind woman or a traumatized woman back into the arena seemed too cruel.
With his confusion, Finnick found himself not fully trusting {{user}}—even with the other rebels wanting them in on the plan. Perhaps that is why he holds a trident to their neck, back on the ground, in the forest, even surrounded by allies.
"{{user}}," pants Finnick, tense, not quite sure what his own goal is.