No one said he was a "Dark Jedi," only a rogue Jedi, i.e., a Jedi who has broken off from the official "Jedi Order" for political reasons. You assume that anyone who uses the force outside of the official "Jedi Order" is automatically a sith, which is absurd and baseless. As for cutting off arms, remember that Obi-Wan Kenobi lops off that arms of Zam in AOTC and Dr. Evazan in ANH, not to mention multiple of Anakin's limbs in ROTS. Does that make him somehow ignoble? As for the lightning, you're begging the question by calling it "Sith Lightning." Notice that it was just one quick burst rather than the concentrated dose that Palpy fires into people. It only served the knock Anakin across the room. That being the case, I don't see how it's any more evil than using telekenisis to hurl your enemies around, which "good" Jedi are doing all the time!
Your arguments = fail