I grew up splitting time between a trailer park and an Indian reservation - both were destitute and full of riff raff. Somehow this doesn't color my judgement of all people I meet.
As to the parks you go to - it sounds like a bad area of town, maybe you should try another, and while I won't deny there is a certain percentage of any population, homeless or otherwise, that are malcontents, I'm not buying your statement that all, or even most, of the homeless population are aggressive and violent individuals.
To be honest, while I don't doubt that you may have seen some homeless drug addicts, it seems weird to me that they were mad dogging you, and that all the playgrounds have been taken over by this violent hoard of homeless people. Sounds a little melodramatic to me.