It was an illegal arrest, it was unlawful to arrest suspects at night in Roman Empire. The soldiers come out in number and well equipped, probably expecting much resistance or a public demonstration. They were thrown back by His I AM introduction. Jesus demonstrates to them how powerful He was and how in control He was; meekness, power in control.
Jesus willingly gave Himself up so He could protect His disciples. After all, He could have just kept saying I AM and walked way!
John 18 “The Great Exchange I (The Arrest and Trial)”
