So the thought now is that the shot was around 200 yards, which some are saying is not a hard shot to make. However, this person took that shot once, killed Kirk with law enforcement and people all around them, apparently packed up and/or disassembled their rifle, including possibly the spent cartridge, egressed from the top of the building without being seen by eyewitnesses, apparently made it to transportation and left the scene while somehow rendering all of the camera footage, personal cellphone footage and license plate readers that LEO relies on these days ineffective (so far).
This appears to have been fairly methodically planned. It doesn't appear to be a random person with minimal firearms training who bough an AR and decided to do this on a whim. This person appears to have been experienced.