Definitely a problem where we live. (Thank goodness our close friends, who don't live near us, have a similar, strict parenting approach to us, so we can still enjoy time with them and their DC). Absolutely no need for shouting or smacking (both low class behaviours IME), just firm, immediate, negative consequences.
We live in a nice, middle class area but see countless examples of primary school aged children allowed to hurt others and cause a big nuisance with literally zero consequences. E.g. 6/7 year old boys rampaging around the playground/softplay knocking toddlers to the ground or breaking equipment and the parents say nothing.
If mine did that I'd be straight over to him, remove him from the area, make him go and apologise to the toddler and their parents (and also apologise to the parents myself) or to the softplay/library manager, and then take them home. Discussion that evening about what they did wrong and why it won't be happening again. Surprisingly enough...my kid therefore never behaves like this, so it's not necessary! Because I actually took the time and effort to supervise and parent him properly aged 2-5 when he was still learning what appropriate behaviour was. Again, no shouting or smacking necessary, just swift, consistent hard consequences.
There's a 6 year old boy on our street I feel really sorry for - none of the other kids invite him to play as he is just a nightmare. His parents claim he has ADHD. No he bl00dy well doesn't, he just has zero boundaries and unlimited screen time! (We know the parents well so have witnessed this in action for the last 5 years). He's allowed to stay up as late as he likes watching tv and playing video games, then his parents claim "school tires him out too much". Again, no it doesn't! staying up until 10pm playing playstation is what tires him out! He broke a door in our house once, by slamming it as part of a "game". Not only did his parents not apologise, they actually did literally nothing, said "oh he has so much energy he needs to let out!" and just let him carry on (we were all sat in the room and watched him do it). I got up and said "right, think it's time for you all to leave as X is clearly struggling to behave himself". Mum looked at me like I'd called her son a swearword! Yep, nobody will like your child or choose to spend time with them if you let them act like a spoilt, violent brat! Time to accept that your gentle parenting approach isn't working...but of course she won't.
No, I don't think children like this will end up carrying knives - I think that's an issue that's particular to more disadvantaged neighbourhoods, with violent parents, drug issues, etc. But these "gentle parented" middle class kids sure as hell won't end up contributing positively to society, or even become well-functioning adults. It's a real shame. I don't know what we can do about it though, it's like a pandemic of bad parenting.