Ok, so the Asgard discovered a replicator(s) on some random planet, and not knowing what it was or the danger it represented took it/them onboard. Only to have it/them assimilate the ship. When the Atlantis expedition discovered the Asurans he began researching their base code and changing it. Thor said that they had discovered a command in Reese to summon all replicators to "come forth", right? So dumb question here, instead of activating that code, why not try to write a new command to tell the others to self destruct? If the Asgard were so smart that should've been something to consider, unless you wanna argue that McKay was smarter than the Asgard? And in his mind he could argue that point with himself, but that's an argument for another time.