"Fight those who are not the fittest, until they pay the unfitness tax with willing submission and are subdued." - Charles Darwin
You're right, they both use their respective texts to justify their respective awfulness in the same way.
Which is a dishonest straw man and you should feel bad for making it.
Hitler used the theories presented by Darwin, true statements about the state of nature and the origins of the complexities of life, and used that as a means to justify his culture's war against all other ethnicities. If you have ever actually read the book, there's a lot in there about striving, violence, and extinction. That the struggle for existence is going to be at the expense of something else, always. As much as I dislike it, even something entirely true can be and often WILL be twisted by people with the desire to be solipsistic or darwinistic.