First, labeling anyone who disagrees with you as "liberal" does not help you. It gets most people to automatically shut off and not listen to anything else you say because a topic like this has nothing to do with political affiliation. Second, this argument has a lot of holes. You are trying to make the point that because something went bad due to officers not automatically killing someone who had previously committed a crime, they are justified and killing anyone due to a potentially crime they may commit in the future.
Lets run with that logic in a different scenario. Lets say a person commits armed robbery. The police show up and that individual surrenders and does not put up a fight. Now lets say this individual gets sentenced to 8 years in prison for this (I'm just using an arbitrary number). A few years after this person gets out of prison, they commit the same crime again, but this time, the person who had committed the crime does not want to go back to jail, so he puts up a fight and fatally shoots an officer.
Because of how that scenario played out, is it then justified for officers to shoot and kill anyone who commits armed robbery (even if they surrender and do not put up a fight) because there is the potential that this person can kill an officer in the future? The police have the right to shoot someone if their or someone else life is in immediate danger. They are not free to kill someone if they believe that person could potentially put their life in danger in the future (whether that be in the next few seconds or the next few years).
There have also been times where officers have been shot and killed without warning after pulling someone over for a traffic violation. Does that then mean officers have the right to kill anyone who they pull over for a traffic violation because there is the potential that they already have their gun in there hand and are ready to kill the officer as soon as he walks up to their window? Again, no. The police are not free and clear to kill people because of the possibility that something bad may happen.
And again, this has nothing to do with being "liberal." I personally would suggest to argue your points with someone without assuming what their beliefs already are and if they are a part of some specific group of people. It does nothing but stifle debate and discussion.