Diotrephes would not submit to leadership, that made him dangerous. Diotrephes was a self serving, glory seeking, person who wanted preeminence. He fail to realize the Only Christ should have preeminence (Col. 3:18). Diotrephes was a slanderous, gossiper. He went around spreading mess about the man of God.
Diotrephes was a divisive person. His selfish desire caused division in the church. Anytime you have a member (s) in the church who displays these attributes, that person is dangerous to the body of christ.
How are we to respond to the spirit of Diotrephes? Verse 11 says we are not follow them because they have not seen God, in other words; they have not been saved. Paul said in Romans 16:17 that we are to mark them that cause divison AND avoid them.
In summary, Diotrephes can be anyone in the church from the pastor to the people. Diotrephes could even be you my reader. Ask God to examine your heart today and see if you are the most dangerous person in the church.