For the record, I know nothing personally of what red flags are being alluded to here. I don't know the guy. I'll just state what I have heard that could be along those lines but you all can judge for yourselves.
For one thing that has been reported is that he was a cook in the National Guard who never was deployed and never saw combat. This was confirmed my the military as reported by the media.
Several Skywest pilots including close friends of his, have come forward and have said that he had mentioned to them at various times over the years that he was a Special Ops guy who saw combat. Where he was telling people he saw combat I don't know.
Is this a red flag? Who knows since a lot of people lie about their military background, their flying careers, their medical background, including the last CEO of Yahoo who said he had a degree he didn't have. Politicians as we know are the biggest liars of all. So you probably can't call this a red flag since lying about something usually doesn't lead to killing your girl.