We can compare them and see that the police force oversteps its civilian role. Your point about the requirements to kill enemy combatants strenghtens that arguement.
My point is that we don't define what police do based on what soldiers do, so it's a red herring in terms of politics and legalities. We define whatever the limits on policing are separately, and tear gas is specially carved out for use by the police. My point isn't that that's right but that it's again not defined by the reasoning used to prevent escalatory chemical warfare in an actual war.