I think it goes back to what Ferretinabun mentioned--if you're going to make a decoy, why make a highly dangerous poisonous gas a decoy? Why not make the decoy a dud, or something that will kill only the person putting together the recipe? I'm sure we could come up with a number of other solutions, but the point is that while it wasn't the Arishok's fault that a thief stole his recipe and intended to use it to murder innocents, he still holds some responsibility. The recipe could have been anything.
And, if you really think about it, presuming that the recipe is an actual piece of paper, there's no reason whatsoever to have it lying around. Someone could have just memorized it. That way, the only way to lose the recipe would be if a qunari deserted or if it was tortured out of him or something. But again, we don't have all the details of the theft.
In the end, though, I tend to blame the arishok more because, to me, it sounded like he let the recipe get stolen. Not only could he have protected the recipe better (because, come on, they're not doing a whole lot everyday) but there didn't have to be anything worthy to steal. Does that make sense? Plus, this isn't a guy who's patiently waiting for a ship to send him home. This isn't even a guy who is trying to avoid conflict, simply wishing to find the relic and go home. To me, the arishok doesn't really care about innocents getting hurt, about the gas killing people, which is exactly why he lets it get stolen. If the qunari were framed for the gas killing people, I think the arishok would welcome the fight that would come and seize the opportunity to get rid of all these inferiors irritating him. Is he actively trying to start a war like the elf? No. But he's certainly not going out of his way to prevent one either. You could even say that, with decoys like the gas, he's egging people on. He's been stranded, or at the very least, unable to go home for years. He's bored and angry, surrounded by inferiors and fanatics looking to start a holy war. Again, he's not actively trying to start a fight, but, if the opportunity arises...