"What about the bad Sufis we read about, who contravene the teachings of Islam?
The answer is that there are two meanings of Sufi: the first is "Anyone who considers himself a Sufi," which is the rule of thumb of orientalist historians of Sufism and popular writers, who would oppose the "Sufis" to the "Ulama." I think the Qur'anic verses and hadiths we have mentioned tonight [see article] about the scope and method of true Tasawwuf show why we must insist on the primacy of the definition of a Sufi as "a man of religious learning who applied what he knew, so Allah bequeathed him knowledge of what he did not know."
The very first thing a Sufi, as a man of religious learning knows is that the Shari‘a and ‘Aqida of Islam are above every human being. Whoever does not know this will never be a Sufi, except in the orientalist sense of the word—like someone standing in front of the stock exchange in an expensive suit with a briefcase to convince people he is a stockbroker. A real stockbroker is something else.
[Important para]Because this distinction is ignored today by otherwise well-meaning Muslims, it is often forgotten that the ‘ulama who have criticized Sufis, such as Ibn al-Jawzi in his Talbis Iblis [The Devil’s deception], or Ibn Taymiya in places in his Fatawa, or Ibn al-Qayyim al-Jawziyya, were not criticizing Tasawwuf as an ancillary discipline to the Shari‘a. The proof of this is Ibn al-Jawzi’s five-volume Sifat al-safwa, which contains the biographies of the very same Sufis mentioned in al-Qushayri’s famous Tasawwuf manual al-Risala al-Qushayriyya. Ibn Taymiya considered himself a Sufi of the Qadiri order, and volumes ten and eleven of his thirty-seven-volume Majmu‘ al-fatawa are devoted to Tasawwuf. And Ibn al-Qayyim al-Jawziyya wrote his three-volume Madarij al-salikin, a detailed commentary on ‘Abdullah al-Ansari al-Harawi’s tract on the spiritual stations of the Sufi path, Manazil al-sa’irin. These works show that their authors’ criticisms were not directed at Tasawwuf as such, but rather at specific groups of their times, and they should be understood for what they are."
Read complete article (lecture transcription) here. For another short introduction to Sufism, a work that negates or clears misconceptions firsts, and then affirms or elucidates what the matter is all about - (this is analogical to what Shahadah in Islam is, i.e., La ilaha illa'llah) - i find this work useful: Sufism: Principles & Practice by Dr. Hamid Algar.