Harsh and unforgiving, Thread appeared in Catching Fire, ready to enforce the law, no matter what. Being fiercely loyal to the Capitol, he was willing to do whatever it took, going to such extremes that he injured Katniss, the victor of District 12 who had an upcoming modeling session.
Upon arrival, Thread brutally beat Gale Hawthorne, who had been caught poaching a wild turkey (for tackling Thread, stopping him from beating someone in the film). Although Gale had been selling meat to the Peacekeepers of District 12 for years, they turned a blind eye to this illegal poaching, since they were his best customers. Gale went to sell the turkey to Cray, Thread's predecessor, when he unexpectedly found Thread at Cray's house. Thread saw Gale's poaching as a flogging offense, and threatened to kill Katniss or Gale if he saw either one of them in the woods again. Darius tried to intervene after about 20 lashes by grabbing Thread's arm but was struck in the head by Thread, knocking him unconscious.
After that display of cruelty, people stayed inside their houses, rather than risking the wrath of the new band of Peacekeepers, including Katniss. A few days later, however, Katniss crept out of her house and headed to the woods where she met Bonnie and Twill. Upon her return, she found Peacekeepers sent by Thread to deliver a message that the fences would now be electrified 24/7.
When the Capitol firebombed District 12, he was transported out along with the other Peacekeepers in the district. Romulus' current status is unknown, as he is not mentioned any time during Mockingjay. It is possible he was executed by Snow during the events of Catching Fire as indicated by Snow when he witnessed via live stream Katniss, Peeta, and Haymitch standing up to him for Gale.
Romulus was ruthless, pitiless, violent and sadistic. He was also cruel and vicious, not caring who got hurt during his military activities. He carried out his orders without mercy and without compassion, making him a somewhat sociopath. Romulus Thread was an idealistic, angry and powerful man who seemed to terrify everyone with his mere presence. He was also very aggressive and declared that anyone caught breaking the curfew he organised in District 12 will be shot on sight.