Confirmation bias is a helluva drug.I don't believe they discussed killing the red dragon knights. Of course that person wouldn't because the plan falls apart once other red dragon knights provide their own testimonies, but isn't it going to ring alarm bells that the red dragon knights have been killed and that the one who gave the testimony is also dead.
Well, they all got riled up and failed to properly question the person making the report and double-checking it, so they're clearly not going to look into these oddities.
hmmmm how do we make the readers sympathize with the demons? Oh I know, let's make the humans cartoonishly evil and dumb