And why was Sandra Bland murdered in the Waller County Jail by Cops?

Because it was fun for the cops to beat her (rape her?), then stage her "suicide"--but also because it provides the Powers that Are (the FBI is overseeing the investigation into her death) a useful object-lesson for the perhaps insufficiently cowed citizens of Texas and the United States: That cops, being by definition above the law, can and will murder whomever they want, whenever they want, without let or hindrance.  The bottom line being that in a country without habeas corpus or the rule of law, where torture, confiscation of property by unaccountable authority, arbitrary detention, and murder by the whim of the chief executive are the law of the land, it does not matter whether the people respect authority or not, so long as they fear it.

The interesting thing about this mugshot of Miss Bland obtained from the Waller County Jail is that she was apparently already dead when it was taken--You can see, if you look closely (particularly from the fly-away positioning of her dreadlocks), that it is a picture taken of a corpse lying on the floor, not a photograph of a woman standing with her back to a wall.