Was Peter part of a militant sect? John 18:10 (NLT)  
  
  &nbsp;10 Then Simon Peter drew a sword and slashed off the right ear of Malchus, the high priest’s slave. 


The pastor at a church I used to go to once said1 that Peter was part of a militant sect of Judaism that felt like it was their duty to kill Roman soldiers as often as possible/prudent. This would explain why Peter had the sword with him and why his first instinct was to attack. However, I don't know either the name of this sect or whether he was involved in one.

1: If absolutely needed, I can dig up the sermon transcript online.
There was a militant sect in the first century known as the Kanai or Zealots. One of Jesus' apostles was named Simon the Zealot. Some scholars think he might have been associated with this group, but the best evidence seems to suggest the Zealots as a political organization did not exist until decades after Jesus' time. In any case, he was not the same person as Simon Peter.

A splinter group known as the Sicarii carried concealed daggers to kill Romans and Roman sympathizers. Some scholars have tried to suggest that Judas' second name Iscariot is a Hellenized version of Sicarii, but again the best evidence suggests they were not active until much later.

As for Peter's drawing the sword, the most plausible suggestion I've heard is that he was impulsive and often spoke or acted before considering the consequences, just like when:


he tried to walk on water following Jesus example, but couldn't
he rebuked Jesus for talking about suffering and dying
he promised Jesus that he would never deny him