The scale of horror Hitler's philosophy enabled and created, much of which he authorized or approved, is unprecedented in human history either before or since. Due to this, Hitler has come to be seen by many as a symbol of pure evil.
While it is vital that we learn from studying Hitler -- how he gained power through demonization, the horrendous actions he took, and how those actions came to be seen by many as necessary or even good -- it has also become an easy rhetorical tactic to use comparisons with Hitler to discredit individuals or ideas that one dislikes (guilt by association).
One response to the abuse of this tactic is Godwin's Law, which was formulated originally as an observation but has been widely adopted as an informal rule of conduct (especially on the internet) to prevent easy demonization through comparisons with Hitler.
It is also important to remember that, while Hitler is often described as a "monster", there was nothing especially monstrous about him as an individual; his crimes were largely or entirely on a political/cultural level. In person, he was apparently affable and compassionate (to anyone who was not one of the groups he sought to demonize and destroy, at least). He was not a monster; he was a human with monstrous plans.
In other words: there is nothing in Hitler's life-story, up to the point where he began his tyrannical reign, to suggest that he was any worse than many thousands of other extremists, then or now. He was simply an extremist who managed to achieve enough power to actually implement his extreme ideas on a massive scale.
It should be noted that even today, there are many (a tiny percentage of the population, but still a large number of people) who agree with Hitler's philosophy to various degrees -- despite knowing where it leads.