Oddly enough, this is the first place that I've seen this discussed, not even in the news, but I've been thinking about it too.
Disclaimer, I'm all for what the Dallas PD did. That being said, I was Military Police in the Marine Corps and am well versed in when you can and cannot use deadly force - (to paraphrase)
-To protect yourself and others
-To prevent the escape of a dangerous felon
-To protect assets and information vital to national security
So, if the scumbag was cornered, trapped, and not coming out - he couldn't escape - he just wasn't coming out, then how is deadly force justified?