No the ends do not justify the means.
One, know one could know what Hitler would become. Therefore, until he acted in such a manner as to orchestrate the massive genocide he did accomplish we would have no reason to kill him at all. And we do need to remember that he did not accomplish what he did alone. Millions listened to him or he would have failed without even killing one person.
Now, once he and his armies started doing what they did, we would be called to act in defense of the innocent and even ourselves. And this is where we start dealing with what constitutes a “just war” or acts of self defense. The Catechism does allow for one to kill another IF there is no other way to protect society or themselves from such a person. But we are not called to kill at random or for the fun of it, or just because we feel like it…etc. It is a last resort.
Thus I don’t think fighting someone like Hitler, Stalin or the next fool like them, is really an example of “the means justifying the ends.” I could be wrong, but for me these type of situations just don’t fall into that category as I understand it. These are radical and extreme situations that stand outside of reason almost, and we certainly must have God’s help and intercession in such cases.
One could argue that Christians in general could have ignored the insanity of Hitler, but certainly millions more would have died. But could it have been possible that through such a horror Hitler and his followers would have simply ran out of steam on their own? With all the victims left to bury they would have had time for nothing else.
My own choice would have been to fight with all my strength, in anyway possible. I don’t know where that places me in God’s eyes, but I would say my conscience would be clear if I was fighting against such an evil danger as this man proved to be. I have no desire to harm anyone, but I can’t sit by and watch others be harmed either.