That kind of event happens sometimes.

The first reason it’s not more common is that wealth isn’t so static. Landlords have to collect rents, oligarchs have to skim off the top of their corrupt enterprises etc. A guard who kills the landlord, then manages to organize rent collection for the benefit of other guards, has essentially become the new landlord.

The second reason is that a guard doesn’t just have to kill the landlord, a guard potentially has to kill all the other guards who want all the landlord’s wealth for themselves.

Since rent has to be collected anyway, and an individual guard has a low chance of surviving against all other guards, remaining a guard is a lower risk option.