I think such questions are hard to answer in general. I would say a person living in one (small to normal sized) flat and owning + renting another isn't worse than one person 'occupying' just one but bigger livingspace. If an old lady lives alone in a big house where there are sufficient rooms for 6 people+ she's taking away as much property from the market as the small-scale landlord. Sure that's not optimal for society but I also wouldn't necessarily consider that unethical.

If there is a housing crisis in an area, one can argue that short time rentals are evil but also short term rentals are important to some extent. If everything becomes an AirBnb that's obviously bad but I think there's also a healthy amount of that. If a city or region has a lot of tourists or business travellers, they need to live somewhere and traditional hotels don't work for everyone.

From my perspective, there must be a healthy balance of personal livingspaces to buy, for long term rent, for short term rent and commercial buildings. Regulating that healthy ratio should be a task for politicians. Unfortunately, I have to admit that government regulation is not exactly working fine in most parts of the world.