Police officers don't want to kill anyone, but they must take necessary action to neutralize a perpetrator. Sometimes, that may require shooting the person. It is up to the officer to determine, based on the circumstances, what level of force is required, and that is why it is a tough job that most people either can't do or won't do.
A fair trial is not the concern of the police officer. If the perpetrator is neutralized and lives, and is arrested and charged, then the person is turned over to the justice system, where it is up to those involved in administering justice to provide a fair trial, as required.
Liberals combining the idea of law enforcement with justice administration is not surprising, given that the same type of people believe in increasingly centralized government by which an increasing amount of the citizen's life is controlled and determined.