Homeowners invest their own money, at significant interest rates, to obtain a place to live. They do of course have an interest in protecting that investment - with increased density comes increased crime, stressed parking, and other problems.
To call homeowners a "cartel" as if they're extorting their local community for protection money is inherently misleading.
For instance, Canada has a legal maple syrup cartel: https://en.wikipedia.org/wiki/Federation_of_Quebec_Maple_Syr...
Hold prices for their own suppliers artificially low by agreeing a price cap and refusing to pay more. They rely on suppliers having nowhere else to sell. e.g. British supermarkets have been repeatedly accused and sometimes IIRC convicted of operating a cartel to demand unfairly low prices from food producers.
Agree to split geographies so that there's effectively no competition to supply and each member of the cartel has local monopolies in all or most territory. This should be familiar to US folks who want High Speed Internet outside of a major city.
All agree never to offer a product which would hurt the overall market but be profitable for the first to offer it. Some have claimed the long delay in electric cars coming to market is evidence of this sort of cartel.
Cartels are illegal in much of the world (but ordinary laws don't apply to sovereign entities). If you're ever part of some big industry insider meeting and the lawyers make everybody agree not to talk about prices, future products or other commercial issues it's not because they're commercially sensitive, it's because your executives could do jail time if a court decides your group is a cartel.
Increased density does not increase crime. There's no evidence for this. There are very dense places with low crime and very low density place with lots of crime.
"stressed parking" is probably true, but part of the point of increased density is that you have less need for a car.
"other problems" is dog whistle.
I think I agree with you, but you are over simplifying the issue. Increased density does not increase crime per capita. It does increase crime per other measures of crime intensity (crime per square ft?). https://nycdatascience.com/blog/student-works/pressure-cooke...
http://theipti.org/wp-content/uploads/2012/02/covariance.pdf
http://newsinfo.iu.edu/news/page/normal/13030.html (non-primary)
Damn right you're using violence to gain a profit.
When you violate zoning they don't ask nicely you know.