How can conservatives say Hitler was a leftist/socialist?

[Southern Senator North Carolina Yankee]

Because they focus too much on the means and the desired ends. The how and not the why.

[Southern Senator North Carolina Yankee]

Ideology exists by maintaining the illusion of longevity. The longer than can make it appear to have existed in its current form, the more likely it is to curry support and also the easier it is to maintain conformity and suppress dissenting view points.

Therefore, the incentive is to redefine all of history on those metrics that define your ideology to achieve those ends.