Slippery slope?

It isn't a fallacy if you have reason to believe the entity in question has been frustrated in it's more extreme goal.  The fallacy is where you assume something has an ever more extreme objective, that it won't stop when it gets to a certain point.

But in politics that tends to be how things work, entities can't get what they actually want so they support something more moderate to build up strength to ultimately pursue their original more extreme goal.  That is because political actors are typically frustrated in their actual goals by opposition, so their present proposals have a 'slippery slope' character. 

The problem with the slippery slope fallacy is that its speculative, its a hypothetical and its mathematically improbable. The final outcome becomes more and more mathematically improbable with each step thats added in the slippery slope. Its fallacious reasoning because there is so much room for error in this line of reasoning and one cant make a truth claim based on a speculative hypothetical argument. Because at the time the argument is being made the claim hasn't been tested yet. 

Well, it certainly is a by-product of various innate cognitive mechanisms to both seek and find patterns even when it is an illusion. Certainly, most of the pseudo sciences of ancient astrology, haruspicy, soothsaying, prophecy, etc. developed from these primitive instincts. ( see: Boghossian HAAD or hyperactive agency detection, and  Dennett and McKay on "promiscuous teleology in "The Evolution of Misbelief.

However, in logic, we look for the inherent deception in any fallacious claim, as per

Dr. Bo's Criteria for Logical Fallacies:

It must be an error in reasoning, not a factual error.
It must be commonly applied to an argument either in the form of the argument or in the interpretation of the argument.
It must be deceptive in that it often fools the average adult.

The Slippery Slope is both a valid concept and a fallacy (see slippery slope ). I think I did a pretty good job there explaining under what circumstances the slippery slope would be fallacious. In short, it is all about how probable the first link is to lead to the last. The less probable, the more fallacious.