Why did Ozymandias kill the Comedian in watchmen movie?

In the Watchmen movie, Ozymandias (Adrian Veidt) did not kill the Comedian (Edward Blake). It is revealed in the movie that Veidt orchestrated a staged alien attack on New York City to unite the world against a common enemy, thus preventing nuclear war. As part of this plan, Veidt hired a group of criminals to pose as aliens and attack Manhattan, but the Comedian was not among them.