"without full knowledge of the situation". Seems to me that the LEO had as much information about the situation as was possible in the timeframe required to act. Someone, at night, was pointing something at them from across the street during a gun battle with someone else (to be fair that's entirely based on information from this thread so we can treat it as a hypothetical situation if you like). I think it's pretty reasonable that a phone can be easily mistaken for a handgun in those circumstances (not sure why you're suggesting the comparison to an assault rifle).
That demands an immediate assessment of threat. I don't think it's reasonable to suggest that they just needed to wait a bit longer for more/better information to arrive when their current assessment based on the information available at that moment suggested a high risk to themselves or others.
LEO often have to deal with situations where information is imperfect (poor visibility, multiple actors, etc), and make judgement calls about what's the best/safest thing to do in the circumstances. Sometimes they get it wrong, because the information wasn't accurate enough, and sometimes they get it wrong because their training was insufficient or their judgement was flawed.
Clearly if training or poor judgement (or attitude) were factors in taking the wrong action then that should be addressed (and there may be a fair point to be made that in some jurisdictions LEO and their organisations are not held accountable enough for failures there).
And sometimes they get it right, and save the lives of themselves or others because they acted immediately.
Full disclosure: I have LEO family members (Canada), so I recognise I have some obvious bias on the topic.
[edit] looks like 'at night' may not have been correct; not sure where I got that from. I don't think it changes the argument much - information at hand is often imperfect; LEO need to act in some cases regardless of imperfect information due to the level of risk involved.