Since ethics are not considered by utilitarianism, violence could be used in order to maximize utility. For instance, if a nation found engagement in wars lucrative, that nation would be justified to start wars because from the results of war, utility could be maximized. To take this to an extreme, if a nation could prove that it could maximize its utility by engaging in nuclear warfare and completely wiping out other countries, the utilitarian would have to approve of these horrific actions.
A utilitarian could postulate that in order for something like nuclear warfare to happen, the utility created would have to offset the pain caused, which is unrealistic. However, since the notion of measuring pain could be disputed, all one has to do is make a compelling argument showing that pain could meta-physically not be