It also doesn't make any distinction between events that took place where the intended victims were allowed to be armed or not. Of course there will be less instances of armed defenders in areas where arms are prohibited.
OPs premise is akin to the "small government" advocates who ruin government services and then point at how they don't work.
Generalizations that are oversimplified to the point of lacking all nuance are probably untrue because there are bound to be exceptions. Instead, try including 'many', 'most', or such as an easy remedy.
Specifically, landlords can create value when they handle property management and maintenance (and the related costs) efficiently. It is wrong that greed has made that so rare.