In John's time there were already men like this man Diotrephes. The thing that marked him was that he loved to have the pre-eminence. He even opposed the apostle John, for John had written to this church to receive certain men, but Diotrephes either refused to accept John's authority, or rejected John by not receiving those men (cf. Matthew 10:40).
John brought five charges against him:
he insisted in being the leader of the local church;
he actually refused to receive John;
he made malicious statements against the apostles;
he refused to receive itinerant preachers (the reason obviously was that he wanted to do the speaking and teaching himself); and
he excommunicated those who did receive the missionaries against his wishes.
In short, Diotrephes insisted in ruling the church himself. He admitted no interference from outsiders or disobedience to him from other church members. He was self-opinionated, self-made, self-exalting, selff-sufficient, self-willed, self-satisfied, and self-confident. There are lots of "Diotrephes" today, men who put up a very pious front and want to run their church their own way. They generally show little regard for Bible teaching and don't accept all it says, although they may not openly admit it.
The love of pre-eminence is a characteristic of the flesh. The fruit of the Spirit is meekness: this doesn't mean weakness or cowardice. Moses was "the meekest of men", but spoke and acted with the authority God gave him. The Lord Jesus was meek and lowly, but He went in and cleansed the temple.
Diotrephes was attempting to completely destroy the effectiveness of the apostles and especially of John. He was excommunicating anybody who would entertain these men John had recommended
John didn't ask Gaius to tackle Diotrephes: he said he was going to do so himself.
9 I have written something to the church; but Diot'rephes, who likes to put himself first, does not acknowledge my authority.
10 So if I come, I will bring up what he is doing, prating against me with evil words. And not content with that, he refuses himself to welcome the brethren, and also stops those who want to welcome them and puts them out of the church.
11 Beloved, do not imitate evil but imitate good. He who does good is of God; he who does evil has not seen God.