The standard for lethal force is a "reasonable belief of imminent serious bodily injury or death." That belief can be based on the totality of the circumstances, and does not require something direct like pointing a weapon at someone. That belief is also based on the individual officer's perspective and knowledge at the time the force is used. Furthermore, the decision to use lethal force does not require a warning or command prior to using force, unless it is reasonable to do so (my agency uses the words "if feasible" in our policy).

Nowhere, in case law, legislation, or training, is there a standard talking about "clear and present danger."

I do not know how the determination will go for this officer, because there are similar fact patterns to other cases that have sometimes gone in favor of the officer ("justifiable") and sometimes against (with charges brought and even convictions secured). I would say that everyone saying this is clear-cut murder should prepare themselves for a justifiable ruling (what's sometimes referred to as "lawful but awful"), considering the following known facts:

1. The officer was dispatched to a domestic, which is a known dangerous circumstance for both officers and others in the vicinity.
2. The officer goes to the "correct" apartment, meaning the one he was directed towards. Whether there is actually a domestic disturbance at that location or the apartment manager is mistaken is largely beside the point from a use-of-force perspective; as mentioned above, uses of force are judged from the officer's perspective without hindsight, so from his perspective, he's at the place where there was a report of a domestic.
3. The person who directs him to the apartment also shares info that yelling and what sounded like a slap has happened in the past, and that this time "it sounds like it's getting out of hand," which implies that the behavior is escalating somehow from isolated physical violence.
4. The officer, after briefly seeming to listen at the door, likely for sounds of a disturbance, knocks three times and clearly identifies himself twice. There is a voice on the bodycam saying something about the police, which may or may not have come from the apartment, but if so, could lead the officer to believe the individual inside knows that it's the police outside.
5. When the door is opened, the individual in the doorway is holding a gun. While the gun is never lifted towards the officer and it's not a crime to be armed inside one's home, the officer is now face-to-face with a clearly armed individual whom the officer is investigating for a possible domestic which was reportedly getting out of hand just moments before. Furthermore, the time of day is mid afternoon (around 4:30pm), not late at night/early in the morning when it might be more reasonable to open your door while armed due to the shock of someone knocking at a strange hour. It's fairly rare for people to answer their doors while visibly armed during daylight hours.

Based on all of the above factors (and possibly others which haven't been made public), the officer decides to fire on the armed individual in front of him. Clearly, with hindsight, this should not have happened based on everything now known about both the individual (upstanding military record, etc) as well as the situation (no one else in the apartment with him, no ongoing domestic violence situation). However, the officer, by (case) law, cannot be judged against those facts because they were not known to him at the time force was used, and instead the known fact pattern was presumably mostly as laid out above. Those facts alone are similar to many other shootings which have been determined to be justifiable.

I don't know enough to strongly render an opinion, but based on other similar situations I'm familiar with, I suspect the "lawful but awful" scenario will ultimately apply.