It's quite simple. If you are legally traversing through a public area, even for the purpose of observing another individual, you have a right to defend yourself if that individual attacks you. There was ZERO evidence that GZ started a fight. No matter how many times libs try to paint "observation" as a nefarious act, it fails because it doesn't matter. It was legal for him to do so, proven by the fact he wasn't charged for undertaking said observation. The cold hard facts are GZ was well within his rights to mosey down that sidewalk, FOR ANY REASON.
As to your second problem, why should he feel regret for saving his own life? Granted, the taking of a life is tragic, but, taking it to save your own life should illicit no emotional response other than relief.
Your confusion stems from an incorrect assumption that GZ is guilty.