To me, the defining feature of fascism (or communism) is not use of private, politically motivated violence with the tolerance of the authorities. That's bad, but it's not the reasons that I think fascism is bad.
And what are those reasons? Since I really don't see the distinction you're trying to make.
To speak more carefully - violence for the purpose of influencing the "center of mass" of political opinion in a country, when the government is not uniformly in favor of the political position of those executing the violence - is not the same thing as fascism. More colloquially, tactics similar to voter intimidation have been used by fascists, but not only fascists.
My main point was that an ideological label that applies to both FARC and AUC is not a particularly informative label. If fascism is restricted to the usage I suggested, then it is ...
I have found this most wonderful (if fairly lengthy) article, and thought you would enjoy having it brought to your attention.
It is so good, I think we should include it among the references in the "Politics is the Mind Killer" wiki page. But, before that, I submit it to you, and ask you: is there anything in this article that would warrant its exclusion from this site? I mean, besides the fact that it is about politics, and written by a notorious Social-democrat (And is that in itself grounds for exclusion?).