The shooter claimed to have explosives elsewhere in the city that he would remotely detonate. In that situation, lethal force is absolutely justified.
What if he had some sort of system where he has a personal transponder hooked to a dead man switch that triggers when the signal stops?
If someone told me they had bombs all over the city yet had not detonated them after being involved in the shootout as they were, I might wonder at the veracity as someone like that would surely have set them off already?
I guess it's easier when there aren't innocent lives on the line to follow the lines of logic I've proposed, but they seem completely natural and realistic.
• Attacker doesn't have bombs, police blow him up: Dead attacker, everyone else OK.
• Attacker has bombs, police blow him up: Dead attacker, everyone else most likely OK. (Only not OK if attacker has a certain kind of dead man's switch.)
• Attacker doesn't have bombs, police try to take him alive: Maybe dead attacker, maybe cops hurt/killed.
• Attacker has bombs, police try to take him alive: Maybe dead attacker, probably cops hurt/killed, probably civilians hurt/killed.
A dead man's switch is very unlikely, because it's hard to make reliable. If not set up to allow for intermittent interruptions, it's guaranteed detonate explosives early. If it's only set up to transmit "detonate" when let go of, its destruction can prevent detonation. Also, the shooter would have mentioned if he actually had a dead man's switch. I'm betting he didn't think of that.
It wasn't likely that the shooter had explosives, but considering the possible outcomes, incapacitating him as quickly as possible was the best of the crappy choices available. I think the cops made the right decision in this case.
Are you so sure that you'd be willing to bet the lives of potentially hundreds of innocent people? The Dallas PD wasn't.
It's easy to be confident now that we have more information. At the time, the guy had already shot a dozen cops. He had body armor. He'd held off SWAT in a firefight. All actions that attested to his preparation and training. His claims about bombs were quite plausible at the time. Likely exaggerated, but still plausible.
Let's say the police thought there was a 1 in 50 chance that he had planted one or more bombs. Would you want them to risk a 2% chance of potentially hundreds injuries and deaths just to take the guy alive? I wouldn't.