The RP is set in the future. Children can now be genetically modified to be exceptional at one skill, such as stealth, fighting, manipulation, etc. If you piss off the wrong person, or are chosen by random, your child will be genetically modified by a group of people, looking for assassins. The children grow up, being trained how to kill. It is the only thing they know how to do, and they don't question this. Once they hit a certain age (18), they're sent back into any moment in the past to kill. People pay a large fee for this group of assassins to kill.
Eventually the kids will start to question if what they are doing is morally correct. While this RP will certainly center around action, it's also going to be a thriller with a growing, suspenseful story as it unfolds.