There are a few reasons why anti gun advocates and a politician might speak out against making someone a hero following an event like this.
I didn't click on your video, nor do I want to because its painful to me to watch somebody get shot, but I saw a part of it because a gun advocate posted it streaming on twitter where it just came up. I pointed out to them that on average, carrying a gun creates more danger than safety. That's backed up by statistics in studies.
The robber, gun pointed at a downwards slope, not with authority, likely demanded money (there was no sound but that seems likely). The plain clothes cop shot him.
Pros - the robber didn't get a chance to shoot anyone. A potentially dangerous situation was de-escalated quickly.
The plain clothes cop didn't even appear to say "drop the gun" or attempt to negotiate a non-lethal outcome. Given that the robber's gun was pointed down, one could make the case that "drop the gun" would have been a better first option as opposed to shooting.
Shooting someone holding a gun doesn't always equal safety. They can have an immediate rush of Adrenalin or anger and shoot back. This boy with the gun appeared to not want to fire it. He was even pointing the gun downwards. If he'd been a truly dangerous or mentally deranged criminal, the outcome might have been considerably different.
There could be class issues. It could be seen as a middle class cop shooting a poor and desperate kid who only wanted some cash. I have a friend from Brazil and I know that there are some pretty serious class issues going on there. A political representative of the lower class might feel it's their responsibility to say "don't praise the cop who used lethal force. OK, she did her job, but don't make her a hero, the boy wasn't even pointing his gun at anyone". Now, I can't load your link. Maybe your video is gone, maybe it's my computer is lagging, but there are valid reasons, especially in a nation where there's some conflict between the poor and the police, where you can see why a politician would say "don't make the cop a hero".
So there are a few reasons why a politician, or a concerned citizen might raise issue with calling this woman a hero.
As for why some anti-gun advocates may be agreeing with that politician (and you didn't provide any good evidence of numbers behind that). One reasons is because to some, everything is an argument. But others may genuinely feel that praising a "Hero cop" for using deadly force against an apparently very novice and very young criminal, might not be something to encourage.
I've not seen the left using this incident as an argument for anti-gun. But I have seen the right use it for pro carry laws. I'm not sure why you would focus on a narrow slice of the left and then in your comments, try to make it about the entire left, but you seem intent on doing that.
Not all liberals are anti gun. See here: https://www.politico.com/magazine/story/2017/11/04/liberal-gun-owner-enthusiast-second-amendment-politics-2017-215784
I'm not anti-carry, so much, provided the carrier is well trained and the licensing is thorough. My primary objection to gun laws is the right for a person to buy rapid fire, or bump stocks, and not require any kind of license or testing. Several of the mass shootings have been by people with confirmed mental instability and one with terrorist links, and the NRA defends the right for those people to buy semi-automatic weapons with multiple bullet magazines.
Left? Right? We should be talking about laws and weighing safety concerns and looking at studies. Not assigning viewpoints to the left or right but we should be trying to find a fair middle ground and we shouldn't draw conclusions based on a sample size of one event. (though, obviously, people are an emotional species and often do exactly that).
On the risk of carry laws: