Some kind of necessary
There were so many ways to handle Hitler after he came to power that might have mitigated his impact that it seems problematic, at least, to countenance preemptive assassination.
World War II, with all its horrors, was in certain ways an inevitability. Although the pledge of "Never again" has long since fallen away—massive atrocities, brinkmanship in the Cold War, and the unwritten future stemming from the War on Terror—humanity has witnessed and endured a terrifying countenance of evil. The numbers were bad enough as it was; I shudder to think what they would be in other contexts.
The Hitler legacy is unfortunate, but representative of a lesson necessary for the success of the human endeavor. If only we would take the time to actually learn it.