Arachnarok_Rider: You are misinterpreting the apathy, I think. It doesn't mean that all individuals are happy to be murdered for their stuff and will not respond to any obvious threat.
Individuals no, but if a boostergang rolls your neighbourhood and not mine I don't go out and hunt down that boostergang, I just give thanks they hit your neighbourhood and not mine - after of course rolling over to your neighbourhood to check they didn't leave behind anything valuable I could take.
As for Maelstrom, they're not the most organized group, but there's being toasterheads with a few loose wires and then there's letting single mercs walk all over them.
A poseurgang isn't so much a gang as a subculture. The closest real world example would probably be Anon or similar groups that emerged from 4Chan. There isn't one over-arching Maelstrom, Maelstrom is just a bunch of people (and in a city of 10 million, that bunch numbers in the hundreds of thousands) who happen to share similar tastes and views. Some of them form gangs, because that's what you do in the bad parts of Night City. There's no unifying organisation there though; a Maelstrom gang will usually attack other gangs for much the same reason mods fought rockers, but they're just as likely to be feuding with the Maelstrom gang in the next neighbourhood for saying blue LEDs are better than red LEDs.
In terms of the 'response' you talk about, if you could hang around Maelstrom bars or view their chatrooms you'd probably find you were the subject of a few hundred conspiracy theories ranging from being a deluded serial killer to being an inside job, corpo stooge or even fake, depending on the particular politics of whoever is making the assertion. Undoubtedly some Maelstrom would want to hunt you down to demonstrate their strength or similar, but it's equally likely others would want to use you to improve their own position, or simply go to ground and hope to escape your notice. You wouldn't be the first serial killer or vigilante to target a specific poseurgang, you won't be the last.
Frankly, it seems to me that you're trying to find some form of rational explanation as to why the completely broken AI and reputation system actually "works" from a lore perspective.
I'm going from the sourcebooks. While there's a few things that could be pointed at as a departure from how it 'should' work, the way the gangs are presented isn't one of them, at least in the generic sense. They could of course have had you dealing with specific gangs within the various poseurgangs and had you build or lose reputation with them, but that's getting into the narrative as much as the systemic elements, and my assumption would be they didn't go down that route because it simply didn't fit with the story they wanted to tell.
Such missions directly contradict your interpretation of lore that murdering everything is quite fine and nobody cares.
Missions where someone paid a fixer for something to happen, that ultimately boil down to you murdering someone for money. Not entirely sure how that contradicts anything; if anything it would pretty much demonstrate it. If the cops wanted to arrest you for murder, they'd only need to call one of the fixers and ask for a copy of their receipts.
The whole concept of cyberpsychos that have to be put down
So you're saying cyberpsychosis is real and Max Tac are simply protecting the public? How incredibly .. trusting of you :P