There is a long chain of events here. Plenty of sliding doors to consider.
Had not the car been parked in a disabled space, the outcome would have been different
Had not Drejka decided to berate the driver, had he minded his own business, the outcome would have been different.
Had the bystander not decided to alert McGlockton of the situation in the car park, the outcome could have been different.
Had McGlockton not rushed outside to defend his family, the outcome could have been different.
Had he not shoved Dreika, the outcome could have been different.
Has Dreika not fallen to the ground, the outcome could have been different.
Had Dreika not been armed, the outcome could have been different.
Had McGlockton been armed, the outcome could have been different.
All of the above are hypotheticals. They did not happen.
What did happen is that an argument about misuse of a parking space set aside for handicapped drivers led to the death of a man.
The shooter is relying on the SYG statute. He may be right, but the matter must be tested in court, otherwise there will be many more deaths like this by people who choose to provoke arguments with intent to shoot someone.