A friend of Billy Joe Cagle -- the man arrested Monday at Atlanta's major airport for allegedly threatening a mass shooting -- is now talking to TMZ ... revealing the red flags that pushed him to reach out the night before cops swooped in. Nick Roberts joined us on "TMZ Live" Tuesday ...  telling us he hit up Billy on Sunday night after spotting some troubling Facebook posts ... ones that, at first, looked more like warnings of self-harm than threats against others. Nick said during that chat

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