In response to use of deadly force by police officers, it's important to remember these officers are responding to situations they did not create. The people officers have to deal with are sometimes irrational, have behavioral issues or are under the influence of drugs or alcohol.
A person who threatens an officer with a knife or firearm is not behaving in a rational manner. A person who will not comply with an officer's attempt to conduct a routine arrest leaves the officer few options.
I've heard people suggest an officer should shoot someone in the leg or arm and not use deadly force. This is unrealistic for a number of reasons. Persons who are under the influence of alcohol or drugs can continue to be a threat to the public, themselves and the officer sometimes even after a Taser or a firearm is used. PCP is a good example. The officers are required to take action and stop the threat. Officers who do not act would be seriously criticized for inaction.