You can get an ought from an is by means of the law of identity.Once one knows the nature of an object, then by the law of identity, one will know both that it will have and that it ought to have certain characteristics and actions when confronted by specific context.If one knows the nature of man,and facts of reality, and the requirements for man's survival and life proper to a rational being, then one knows what man ought to desire if he wants to survive and live a life proper to a rational being. This is how it is possible to know that libtardism ought to be avoided as much as humanly possible.
Slavery is evil
Taking the fruits of a human’s labor is evil
Collecivism takes the fruits of a human’s labor
Therefore, collectivism is evil
Modern Liberalism (European and North American in the 20th and 21st centuries) is a form of collectivism.
Therefore, Modern Liberalism is evil.
...Or something like that.