Assasains are a type of enemy in Survival Crisis Z.
Assassins are send only if you have -250 friendship with either Rebels or SWAT. They will still be attacked by the zombies and other enemies (even each other if both Rebel and SWAT assassins appear). They carry guns related to the faction you are enemy of, but aren't too fast. They are usually dressed in night vision goggles, ski mask and a trench coat, although there are some variations depending on the faction.
They appear out of nowhere and shoot you on sight. You can outrun them (still running of course so they won't shoot). You can run through a horde of zombies (make sure there's a gap you can run through without taking damage) and let them take care of him or deal with them manually.