If the guy was pointing something that looked like a firearm, shooting first was warranted. As it is it looks more like a situation where tasering would have been adequate and effective.
these "unarmed black teens" are always criminals up to no good
Even were that all found to be correct, the issue here is the appropriate or inappropriate use of deadly force. Although in Texas, an individual can use deadly force in protection of his possessions (say if a car was stolen), a policeman is not that person.
To understand what proper conduct for a policeman is w.r.t deadly force, look at their code of conduct, for starters. Take, for example a policeman who finds the unarmed black teen rummaging in the store with the broken windows. The cop shoots him, only to later find it was the son of the owner, who found the place damaged and intended to stay and protect it.
No, unarmed black teens are not always criminals up to no good. That's a ridiculous statement, I must have passed by or talked with thirty unarmed black teens today. Somehow I never thought that they were all "criminals up to no good."
In fact I don't think any of them were. Why would you?