I wouldn't call a few minor infractions a "sordid" past -- and certainly not a history that by itself warrants the death penalty. Otherwise I agree with you: this is a worrying trend, and everything in the officer's testimony is meaningless by itself until/unless there is more corroborating information. Anecdotally, this story comes to mind: https://www.dallasnews.com/news/dallas/2017/04/30/officer-fatally-shoots-15-year-old-balch-springs
Also, with Walter Scott where the cop tried to place the taser on him after the fact.