Who is this Diotrephes? Perhaps a better question is, "Who does this Diotrephes think he is?" Was he an apostle? Was he an evangelist? Was he a pastor? Was he a leading man in the congregation? Was he an "ordinary" member? John does not say, but it is interesting that John mentions that Diotrephes just loved to have the preeminence among them. It almost sounds as if he was only a member of the church or perhaps an elder. We do not know.
One of his most marked characteristics is he liked to be "Number One." He had to be the important guy, the one everybody came to for answers to their questions, the one to make the big decisions. He even went so far as to say malicious things against John - one of the original twelve apostles. He prated against him with malicious words. He spoke down on him.
John was the disciple that Jesus loved, and here some little man, probably in the church at Ephesus, was talking against the apostle who had put his life on the line for the church many times, who had spent years in exile on the Isle of Patmos, who (tradition says) was put in a vat of boiling oil and was not harmed a bit, a man whom God was obviously with - and this Diotrephes thought he was so important that he could point out John's flaws to the rest of the congregation.
Then he started disfellowshipping people because they did not agree with him. He kicked people out of the church who wanted to fellowship with their brethren whom he had put out. John promised, "When I get there, I'm going to take care of this. I will call to mind all these things and make what this man is apparent."
Given the way he treated the congregation, Diotrephes was a "Satan in the flesh." What he did was evil, which is what John writes in verse 11: "Beloved, do not imitate what is evil." He is warning, "Do not imitate the actions of this man, Diotrephes. He is doing exactly what Satan did."