Riordan did not reveal anything necessary or need-to-know at the time when there were compelling reasons not to trust Loghain. Alistair doesn't know what it takes to kill the archdemon. We, as wardens, do not know what the dark ritual is for and Morrigan does not give compelling reasons to agree with her other than emotional ones. Riordian did not offer any information to counter Alistair's emotions at Landsmeet.
And I mentioned that there were non-metagaming reasons and did not list the clearly metagaming information about killing the Archdemon and the dark ritual. The reasons given are ones Alistair can also see even though he does not find them enough and you may or may not find them enough. The point is that, unlike with Kaidan/Ashley on Horizen, Alistair has the same information that you do at the Landsmeet not whether his choice was valid.
Grey Wardens are full of thieves and murderers and people who got conscripted against their will. Anders left at the end of Awakening while still being a Grey Warden, but he doesn't get crucified for it the way Alistair does.
What does GWs not being paragons of virtue have to do with it? The GW take anyone as long as they are willing to fight darkspawn and, more importantly, deal with the bloody Blight when it's occuring. The issue isn't that Alistair isn't a saint. The issue is that he leaves during a Blight. I already said that if Alistair had left the order but stayed to fight the Blight I wouldn't care. If Alistair had stayed with the order to fight the Blight and then ditched it afterwards, that would have been fine. If Anders had been a Warden during the Blight and ran off I would have just as big a problem with that as I do with Alistair leaving during the Blight. Anders leaves AFTER the Blight and so it doesn't matter. It's not favoritism or holding Alistair to a higher standard than any other Warden during the Blight. No one complains when Alistair leaves the GWs after the Blight ends in certain epilogue slides, do they?
If Zevran, Oghren, Sten, Morrigan, Wynne, Loghain, Leliana, Dog, Shale, Nathaniel, Sigrun, Velanna, Justice or any other NPC had been Wardens during the Blight and tried to leave for whatever reason, I would be just as disgusted by this as I am by Alistair's actions. It's not because it's Alistair that I have a problem with him walking out during the Blight. It's the action of walking out during the Blight for WHATEVER reason that I have a problem with.