Security Deletion is a Diversion where you have to kill a specific enemy. Once you start it, go to the marker and wait for the enemy to appear. These enemies are tougher than normal ones, as not only do they have some super powers, but they also have a lot of health.
When doing these Diversions, sometimes there will be decoys of the target you need to kill. These decoys do not have as much health as the actual target, although they do have similar super powers. Note that if you go straight after the real target, the decoys will also join in the fight, so it is a good idea to take them down before killing the real target.