That hardly justifies it. There's no excuse for shooting at someone who poses no clear and present danger, and who is running away from you.
Actually, back then, I think the law allowed police to shoot fleeing felony suspects, whether they were armed or not.
Most of the limits on the police we enjoy today are products of mid to late 20th century legal precedents and the US Supreme Court extending the Bill of Rights protections to the states.
For another example, Corcoran walking into the rich guy's house and basically stealing the cane as evidence was allowed under state law until, I think, the 20s or 30s. It was only prohibited on federal cases. In fact, the feds used to get around it by asking local cops to obtain the evidence and then turn it over.