instigator

instigator    [In-sti-gey-tuh]

noun.  someone who initiates something, often troubles

"Firearms were the second leading cause of death in 2014 for American children between the ages of 1 and 19. Does allowing gun play by a kid  somehow increase the risk that the kid will one day become a victim or an instigator of gun violence?"

"New York Jets wide receiver Brandon Marshall, who witnessed the Geno Smith-IK Enemkpali altercation last week, said that Geno Smith was being unfairly portrayed as the instigator."

"A police officer should not be the instigator of a crime."