The slippery slope fallacy is not just the argument style, it is when the argument style is used by the event being held up as a cause is not justifiably believed to be likely to lead to the cited effect. (It is an informal fallacy, rather than a deductive fallacy, and, as such, requires evaluation of evidence, not mere shape of the argument.)
Also, neither deductive nor informal fallacies mean that the conclusion of an argument is wrong, in any case, so the conclusion of an argument being right does not disprove (or even provide strong counterevidence) that the argument contained a fallacy. Fallacies are about whether and to what degree a conclusion is supported by the reasoning (and evidence, in the case of informal fallacies) offered to support it, not about whether or not it is true.
If we do X, Y becomes more likely. Y is bad, making a bad thing more likely is bad, therefore doing X has a bad consequence.
That isn't a fallacy at all, it's just an argument that requires you to establish its premises, like all sound arguments. People call it a fallacy as a pejorative when they want to dismiss the legitimate concern and shut down the debate even in the cases where the premise is correct.
Also, neither deductive nor informal fallacies mean that the conclusion of an argument is wrong, in any case, so the conclusion of an argument being right does not disprove (or even provide strong counterevidence) that the argument contained a fallacy. Fallacies are about whether and to what degree a conclusion is supported by the reasoning (and evidence, in the case of informal fallacies) offered to support it, not about whether or not it is true.