While I doubt she killed Gannon on purpose for revenge/retaliation/to hurt AS (and/or LH), I do think it's possible she took her anger and stress/negative emotions (toward AS and the developing marital situation, feeling rejected if he was really filing for divorce, feeling exposed if he was starting to see the light/red flags, maybe desperate, worried about finances/supporting herself, etc.) out on Gannon and just went too far. I'm also not ruling out the earlier theories that the runaway setup may have started out as a ploy to bring Al back to her (position herself as his comforter, etc.) and just went too far.
ETA: Never mind my last comment - I forgot about the bloody board
Scratch the runaway ploy theory...