I applaud Laius tentative, it was much more believable as a speech than the "friendship conquers all" crap that we see always, but clearly, he forgot to account for how far gone Thistle's mind is.
I dont blame him for trying too, he had very few ways of resolving things from this point on, he simply doesn't have the magic or knowledge to undo the seals or even resurrect his friends, and more than anything i think Thistle wasn't being malicious here. Like i said and has been shown in the story, his mind simply is too far gone and he can't see or deal with things without being extreme.