I would not bother with killing Hitler. I would do some research to identify what company can fairly be considered the first commercial advertising firm. Then I would go there and murder everybody on the premises with spectacular, headline-grabbing, blood-on-the-ceiling brutality. I would leave a placard saying, "SO PERISH ALL ADVERTISERS. DO NOT ATTEMPT TO REVIVE THIS INDUSTRY." I would repeat the process exactly on anyone who disregarded that warning.