I think Zareck's motivations are based on decades of hatred towards the Colonial governement, any Colonial Governemnt, from his early days as a terrorist and turned all the more bitter through his imprisonment and later impotence on the Quorum. As for Gaeta, I think he's turned blinded through self loathing and guilt for handing over the names of so many resistance members to 8 on New Caprica, sending them to their deaths, having previously believed he'd saved so many people. He's so full of self hate from the realisation he was so easily decieved and that he has blood on his hands in the same manner as the blatar he came to so loathe, now realising that they share that common element in being duped into aiding the killing of their peers, that the hatred and distrust now spills over towards everyone else.
As for Adama and Roslin, yes they are tired, and shouldn't they be after leading so strongly for four years, holding together a fragmenting, desperate gaggle through so many disasters? They couldn't have known that they'd find earth intact, but they set off never expecting to find it at all, remember? "It's not enough just to live. You need something to live for. Let it be Earth." They didn't fail the fleet - they succeeded against immeasurable odds, but the sense of loss by the fleet's people would not initially be that objective - it'd take time to recover from the blow. It's disappointing to see them weaken, especially the magnitude of Roslin's implosion, but given that she felt convinced she was being given divine guidance and found a burnt out cinder, then perhaps she would feel more than Adama that all was for nought.