Justinia broke the Accord, which is the issue he's bringing up, which allowed Lambert to claim it null and void. As such, he was able to go about his duty as Lord Seeker and protect the people from mages. Whether that is right or not is debatable, but Justinia actions caused this.
Actually, I think Justinia didn't break the accord and the problem was that the templar/mage war would've happened no matter what she did due to extremists on both sides wanting a war to develop. Did her decisions help cause the templar/mage war to start up? Yes, but she was more the minor party in this as the war was pretty much caused by the extremists like Merdith, Lambert, etc.
Justinia wasn't advocating for the Mages to disband the Circles she was advocating for them to meet up to decide it. Templars cracking down on the mages too much was what caused the templar/mage war and even then the vote was practically 50/50. If Lambert had listened to Justinia, do you think that the mages would have voted to disband the circles? No not really as most mages just wanted more rights and freedoms which in some Circles was sorely needed.