Diseases did not conquer hundreds of tribes.

Disease played a major role in the European's ability to conquer those tribes. It's not an either/or situation. It is true that the "Americas" that the English started colonizing had already been devastated by the contagions brought by the Spanish. The English undoubtedly would have found it far more difficult and maybe even impossible to conquer those hundreds of tribes had they not first been so severely depopulated by pandemic. Acknowledging this does not absolve or even lessen the atrocities committed by the English.