Everyone needs housing to live, and housing is increasingly being treated as an investment vehicle by the rich. In many markets, this has decoupled the monetary value of housing as an investment from the use-value it provides normal individuals, causing home prices to increase rapidly.

To your point, our current economic and credit situations have caused home ownership to be essentially impossible for a large number of people. Since home ownership is one of the primary ways individuals can build wealth, this has made it significantly harder for the average family to build wealth - trapping them in debt, making it much harder to save, etc. This is bad for society and for the economy, not to mention inhumane and harmful to millions of families in the US alone.

So, while renting is a necessity in our current economic climate, it is only a necessity for so many due to predatory economic factors preventing them from entering the housing market. Landlords, while necessary in this system, are increasingly corporations rather than individuals, and they are buying up huge swathes of the total available housing - causing increased housing scarcity, pricing more people out of the housing market, and increasing the number of people forced to rent. Individual landlords, as well as landlord corporations, are exploiting the system for profit and either perpetuating the current predatory housing system or (in the case of corporate landlords) deliberately making the system worse for profit, economically harming millions of families and individuals.

So that is why people see landlords as "the bad guy". Whether or not you in particular treat your tennants decently, you are part of a predatory system and are working to perpetuate that system. It is an interesting moral and ethical dilemma because the system we are forced to exist in creates the necessity for landlords (as you said, some people have to rent), but that same system created the conditions that force so many people to be lifeling renters.