You're not really getting it.
I meant tactics as in defending against the initial attack.
They used superior tactics to drive out the Thalmor, and had to sign the treaty because they needed peace. Their war capacity was gone, war exhaustion was through the roof, their major cities looted, no legion above 50%. They needed time, the treaty gave them that time, and war will soon be upon us.
The very idea of killing a king to become king, by walking up to him and killing him, is something only the most backwards of civilizations ever had. It's utterly retarded.
Besides, Skyrim is part of the Empire, and has to live by it's laws. He broke one of those laws, regardless of dumb ancient Nord traditions.