Frankie was a loyal mobster and wanted to help Mike get rid of Roth. its obvious Michael got too greedy and let that power that he was given get to his head. He reminds me of Paul castellano because despite a few guys like Neri and Tom he mostly "ate alone" when it came to other mobsters in his family. Vito was more respected. Anyway getting back to Frankie I guess there was no other option but for him to kill himself. Even though he was royally screwed over he couldn't be a force in the mob anymore. The only reason he broke omerta was because he thought he was whacked by Mike and honestly I dont put it past Mike to do it even though he knew Frank was loyal just to end a war with the Rosatos