Rasta, also known Rastafari or Rastafarinism, is a movement founded in Jamaica during the 1930s. It proclaimed that Haile Selassie I of Ethiopia is a reincarnation of Jesus or Jesus incarnate. Rastaman appears to have no further connection to Rasta beyond his slasher-movie-style theme. It can be assumed via the bag that the Rastaman represents the trope of a serial killer who's also a tourist.
The Rastaman's t-shirt resembles the blood-splattered smiley face from the Western graphic novel Watchmen.