These people are so angry and so bitter their political philosophy is currrently out of favor that they are desperate for something, anything that could put them back where they believe they deserve to be. Like children throwing a tantrum when they can't get their way, they subconsciously pray for anything to cause an upheaval they might be able to exploit and which would justify their position and discredit their opponents.

For the most part, its just wishful thinking, and probably just harmless talk, these people usually have big mouths but little in the way of actual courage; but in any group there are always extremists and zealots who will try anything if they think they can get away with it. For these, sabotage and terror are valid instruments of political persuasion, particularly if they can be blamed on the "other side".

When the dire predictions of what will happen when the "others" take over fail to materialize, they feel the need to give them a little help.

It's important to realize that people like this are not limited to one political group, there are crazies on the right and left. But in my opinion, the ones on the left sneak around in the shadows and hatch their plots in secret, they know what they're doing is shameful and they try to hide it. The ones on the right always seem so bloody self-righteous about it, as if god and freedom were on their side and had given them personal permission.