Not just uncaught murderers, there are a lot of people who have killed without legally being considered murderers.
People who killed people in accidents such as driving accidents or hunting accidents
People who killed in self defense
Soldiers who killed enemy soldiers
Executioners
Police officers who have killed on duty
Doctors and nurses who have made mistakes that accidentally killed patients
Same with America. Despite TV/Movie Copaganda, most cops go their entire careers without pulling their weapons, and certainly without firing it.
The most likely American citizen to have killed is a military member.
‘Being a cop isn’t that dangerous’
You have a whole lot more military than all other countries too making it more likely you meet a killer.