A Bit of Fallacy?
"Man, believing in something and fighting for it?! That's what those people who crashed into the World Trade Center thought!"
"Welfare? Isn't that what Stalin did?"
And so on. Basically, pick X bad group of people and say that they did that too. But let's try another one and see if the fallacy becomes obvious:
"Hitler breathed! You should stop breathing too!"
The fact is that it's not a point of logic that people who we think of as "bad" did only bad things, so that by association only people like that would do X or Y thing. People are complex, multi-faceted entities, and often do many good and many bad things. Hitler was a monster, but the Third Reich provided employment for a previously collapsing nation.
The act in question might be such a good idea that it's just a matter of simple logic. Even monsters could agree with it. Or some similar variant.
What someone has to prove is that there is something intrinsically wrong with the act. There are a variety of ways to do this. For example:
"Hitler killed Jews. Killing is wrong. Therefore, Hitler did a wrong thing." Or perhaps:
"Everyone says nice things. Hitler says nice things. Osama bin Laden says nice things. What matters is actions. So Bush doesn't get away with murder because he has good speech writers." Here, we see a sharp distinction: In one column, everybody fits. In another column, we see that Bush, Osama and Hitler are together, but not anyone else, especially the interlocutors. Further, this is not a matter of coincidence.
It can get even more subtle. So perhaps believing in something and fighting for it, while sounding good, has this intrinsic direction towards making someone intolerant, overzealous, and desperate, such that they are more likely to commit acts like the ones done on 9/11. But then one has to prove that that intrinsic direction clouds out all other possibilities and that there is no way to head a different direction: say, by being compassionate, by holding good values, etc. After all, one could say regarding the 9/11 hijackers that of course people fighting for bad values will do bad things. The rebuttal would be that the 9/11 hijackers could easily have said the same thing, and that "bad values" depends a lot on the beholder. And so on.
My point here is to separate the cases where someone is simply doing a kneejerk from where there is a good analogy to be made.