I am not a landlord, at least not yet!!
In general, however, I don't exactly understand mixing charity with business.
Firstly, having a situation where buying is more advantageous (i.e. the only situation where land-lording ever make sense economically) is a problem. It indicates economic inefficiency.
But assuming you are just exploiting an existing inefficiency (and not actively perpetuating and exacerbating that inefficiency, i.e. doing economic "rent seeking"), I don't get mixing business with charity.
Why should you not extract every bit of rent you can? It's likely more economically efficient, and is better for everyone involved. If a render on fixed income can't afford the market rent, then letting her rent below market is under-utilization of resources.
Now, if you do that because tenant turnover is costly ("who knows how good my next tenant will be? I know 20% of my tenants are shitty, which translates to $X as cost for turnover, which makes it much more advantageous to rent $y below market") - then that is a different matter!!
If I was in the land-lording business - I'd conduct the business as a business, and then do charity outside that, and advocate for better and more welfare spending by the city/state/country.
Now, the irony is that this line of thinking is probably why I will never get to become a landlord. DW basically told me "you become heartless and cold when you do business transactions. So I don't want us to become landlords. I don't like what that will make us become.".