This is the 13976th most frequent Polish word.
postrzelić
To shoot or wound with a firearm.
The verb 'postrzelił' here refers to shooting and injuring the deer as part of an action during hunting by the hunter.
Myśliwy postrzelił jelenia podczas polowania.
The hunter shot the deer during the hunting.
In this context, 'postrzelił' describes the guard's action of shooting and harming the attacker.
Podczas wypadku ochroniarz postrzelił napastnika.
During the incident, the guard shot the attacker.
Here, 'postrzelić' is used in a conditional form, implying the act of shooting someone depending on their actions.