Let's be real: some of the people on drugs or with mental illness can be really aggressive.

Homelessness is not a root cause for things; it is one symptom of fundamental problems that people have. Other symptoms include aggressiveness and unpredictability. These other symptoms may also be at least a partial reason why the people are homeless.

Not all nor even a majority of homeless people developed countries are aggressive or unpredictable, but the risk is higher. Around here most of them are quite tame and humble, at least most of the time, and I'd much rather use the word "weak" than "bad", though.

But yes, homeless people can be scary. Being a stranger in SF, I was somewhat alarmed when driving in the city and finding myself in a central neighbourhood where I wouldn't want to stop at the red light.