How should European colonizers be judged for their treatment of Native Americans?

Some people, especially conservatives, give blanket praise to Columbus and European colonizers, notwithstanding their conquest and displacement of native populations. Those Native Americans are sometimes denigrated as ignorant, brutal, and/or lacking any concept of property -- and hence, as unworthy of the protection of rights. Many others consider the Native Americans either noble savages or at least the rightful owners of the land. They condemn European colonization as unethical conquest or even genocide. Are either of those approaches correct? What counts as a fair judgment of European colonizers in their behavior toward Native Americans? How should European colonizers have treated native persons?