Pretty weak argument. A perp that has killed anyone will have no problem killing anyone else either.
If you have done any research on murders, you would find that most murders are commited in the heat of passion and by someone who knew the victim. Most murderers convicted and in prison are not career criminals, but allowed their emotions to get out of hand with the result of taking someone's life. Those who will kill a cop when running from them are much more dangerous to society then those who kill out of anger in the heat of the moment.