"Satan" is an amalgamation of many gods (and goddesses) of war, death, the cycle of life, and even of nature, love and prosperity. Even the name is a corruption of one borrowed from another culture.
This is not to say that there is not a concentration of unambiguous evil so powerful that it is sentient and capable of independent thought and action, which could meaningfully be equated to "Satan" in the Biblical sense, even if such an entity was not involved in any of the acts ascribed to it in that text.
By unambiguous, I mean that all individuals - even those whose relative morals and beliefs differ 180 degrees from my own - would agree that this was evil. If it were not unambiguous, then my perceptions only really talk about me, not this hypothetical being. It may be that "unambiguous evil" is an impossibility, or that no concentration of such evil could ever achieve sentience, but that's not my specialty and therefore I simply don't know.
By definition, such an unambiguous entity would be bad, if it exists.
I am also not going to say that there hasn't been an entity of some form or other that has interfered with humans for their own purposes. Whether such an entity is good or bad depends on what you believe, though, and so the question becomes subjective.
The Judeo-Christian Satan is a tough one. Some examples maybe a belief in such a being, others may simply be using the name to "name" their fears. In cultures that did not have much abstract thought, naming a fear by personifying it would make some sense. Other posters have talked of scapegoating. In ancient times amongst Semitic peoples, it was customary to load up a goat with confessions and other artifacts representing their fears, guilt and shame. The goat would then be driven out of the town, carrying all of this burden away. The idea of lumping such stuff onto an imaginary named goat would have much the same therapeutic value but without wasting a perfectly good animal.
But did (or does) their Satan exist? That's even harder. If you accept that non-physical sentience exists, then it is possible to imagine such a sentience interacting with early civilizations in a way that they eventually concluded was wrong. They may well have labeled such a sentience as "Satan".
So the question of whether or not "Satan" exists is independent of whether or not the Judeo-Christian depiction is talking of something real or imagined and whether or not unambiguous evil can even theoretically exist. It depends rather more on what you mean by "exist" (does an archetype "exist", since it is theoretical?), what you mean by "Satan", and given the "non-canonical" but contemporary texts, what you mean by "Judeo-Christian" - assuming that you limit yourself to documentation.