It's fascinating that Freddie Gray's killers are charged with -- among other more serious crimes -- false imprisonment. I gather that the point here is that the prosecutor found that they arrested him, confined him, drove him around, etc. without probable cause.
I have never heard of a police officer being charged criminally with false imprisonment for detaining someone without probable cause. Maybe it's happened, I don't know, but it's not standard. Of course lots of arrests are made without probable cause. Most don't end in the death of the person arrested.
Obviously if cops murder people they should be charged with murder, and if cops are occasionally charged with murder, that might deter some murders.
But I suspect that, if cops were regularly charged with false imprisonment for detaining people without probable cause, that would deter a lot more murders. As well as a lot of bogus arrests of people who would otherwise be inconvenienced, humiliated, terrified, injured and traumatized, but not murdered.