Exactly. Vito told Michael that he would be invited to a meeting and then killed. Because Moe told Michael "I tocked to Barzini. I can make the deal with him, and still keep my hotel!", Michael had reason to consider Moe part of the plot to kill him and to view him as a mortal threat. Remember that Moe, like his real life counterpart Bugsy Siegel, was himself a gangster--to Michael he said: "I made my bones when you were going out with cheerleaders."

This is why Hyman Roth's speech in Part II, where he discusses Moe's death and displays obvious anger toward Michael for having caused it, rings rather hollow to me. If Moe's and Michael's positions had been reversed, Moe would have had Michael killed--and so would Roth in the same situation. Michael may not have been justified in killing Moe in the eyes of the law, but he certainly was by the accepted standards of the world these people lived in. Moe was certainly not some innocent victim.