It's always hard to judge distance in these videos but the suspect seems to be approaching within 21 feet when the officer made the decision to fire.
I've seen lots (maybe too many) body cam videos and I can't imagine officers have a very high opinion of the efficacy of a taser. It's frustrating to hear people ask "why didn't they just tase him" when the device has such a high failure rate. It's one thing to fire 2-3 rounds of live ammo, stay in high ready, and evaluate the need for follow up shots. It's quite another to deploy a taser, wait to see if it worked, then transition to lethal force if it didn't.
The detractors and plaintiffs seem to think that cops minds work faster than other humans. In this case, if officer #2 had deployed a taser, and it didn't connect, the suspect could have carved up officer #1 like a Thanksgiving turkey.
Also, I have rarely seen such excellent communication between the officers and over the radio. Very professional.