In some cases assasination is right
assasination is never right EVER
Tragically yes, targeted killing is sometimes indicated. Like most things in life, it has its benefits and drawbacks.
If you notice something good in yourself, give credit to God, not to yourself, but be certain the evil you commit is always your own and yours to acknowledge.
But that deals with only your example. History can't be changed. The assassination of Arch Duke Ferdinand started WWI. I wouldn't rule assassination out, but I also doubt I would ever use it. Assassinating Pol Pot before he killed 2-3 million of his countryman would seem like a good idea, but there is always a number two who probably would have stepped in and continued it. Maybe even more ruthless. It is something to think about.
Early voting in Georgia. On the 20th of October this old Goldwater conservative voted against both Donald Trump and Hillary Clinton by casting my vote for Gary Johnson. Neither Trump or Clinton belong within a million miles of the Oval Office.
Idealistically, no. Pragmatically, maybe.
"The side that stays within its fortifications is beaten." ~Napoleon
I wholly agree with your first paragraph. The only caveat is that you can't predict the future, so what we see in hindsight as a string of linked events we ourselves might not have seen had we been alive at the time.
As far as your second paragraph, I believe it's entirely possible (but not certain) that many of the atrocities were not instigated by Hitler himself, but rather by underlings trying to gain favor with Hitler. If Hitler was the mind behind them, then he did a very good job of publicly and historically disassociating himself and not leaving evidence as such.
If, when defending your support for Donald Trump, and your response is,
"But but but... HILLARY!!!", then you lost the argument before you even began.