In my experience around Cape Town, it's both: High security (or more commonly electric fencing in affluent neighbourhoods) on a single house means the house is safe (or the owners are paranoid). High security on every house in the neighborhood means high crime rate in the neighborhood.
It also don't think crime stats went up much over the last decades in general (may be different in specific neighborhoods), but there is definitely more fear of safety that leads to more security.
I agree that the perception, (and implied acknowledgment) of violence is a very interesting factor — and I would not be surprised if the very act of building a lot of visible security features near a building overall increases the rate of some sorts of crime.
That said, you’re a little out of date on SA - violent crime across the country was relatively stable for a decade or two, but since 2021, violent crimes across SA have spiked, and now even Cape Town is in the 20 most dangerous towns in the world, and in the top 10 murder / capita towns in the world.
Murder rates are now around half of what it was 30 years ago, while at the same time security around homes went up significantly (and most new developments are security estates). The increase in murder rate over the last 3 years is not significant on that scale.
It also don't think crime stats went up much over the last decades in general (may be different in specific neighborhoods), but there is definitely more fear of safety that leads to more security.