Professional shooter...

Here is why...

To recap:

Average Joe does all of this completely unassisted and without advance knowledge of location.

-Walk through campus with disassembled rifle undetected.

-Knows when and where to enter building to best avoid detection.

-Knows which stairs provide roof access.

-Knows that roof access doors arent alarmed or locked.

-Gets on roof.

-Dresses in clothes that mimic roof coloration to dimish profile visibility on roof.

-Assembles rifle.

-Moves into shooting position undetected.

-Knows that there are no line of sight issues on chosen roof.

-Gets in shooting position and makes shot adjustments for weather, line of sight, Kirks sitting elevation, audience, others on stage with Kirk and any unexpected shot impediment.

-Takes more technically demanding shot (not center mass) but head/neck shot because he knew Kirk wore ballistic chest protection (a theory of shot bouncing off chest protection being floated but hard to conceive down angle shot being directed upwards)

-Gets off roof.

-Initially avoids massive police response and escapes university.

Yeah sure… anyone average joe could do all of that by himself