Criminals are people too. Your distinction would apply if the headline had "non-criminals" (a rare construct, but for sake of comparison, akin to "civilians" or noncombatants) instead of "criminals".
The geography of Rio's slums makes killing innocent people rather easy even if you don't set out to kill them; however, there's significant paramilitary activity in the area, both as the drug trade groups you mention, made up of civilians, and groups of officers and ex-officers who initially ran protection rackets, but now run drug operations too. These officers are often high up on the police hierarchy, and use the police structure for their own goals.
(As a reply to another comment: fascism and various genocides have been asked for by citizens throughout history; public approval doesn't make a policy good or moral per se.)
An example of what their "judge, jury and executioner" mandate can bring, aside from direct executions: https://www.latimes.com/world/la-fg-brazil-rio-militias-2019...
I'm on the fence about this as I don't have enough information to form an opinion. I'm European living in the US and so far removed from anything happening in South America that I cannot judge based on the news. I have one Brazilian friend who does agree with this approach so that's that.
Bun in general, this is going too far. It's no longer a slippery slope. If the cops have authority to fly around and snipe 'criminals' then the opportunity for abuse is ridiculous. Nobody should have that power.