Does something immoral become moral because a disabled person is doing it? Wouldn't tricking an able bodied/minded landlord into giving up their property be good? Or do you think we have to just be better than our enemies at all moments?

The weird thing about morality is that depending on the circumstances, the same action can become more or less justified.

An able-bodied landlord, at worst, would still be able to get a job like the rest of us. This man is entirely dependent on the income the property provides and has no way to augment his income at this point. The stock market is also unethical, would you feel the same if his 401k got signed over to someone? The answer is the dissolution of these predatory means of ownership while ensuring a good standard of living for everyone. Not taking an disabled, eldery man's main source of income for your personal gain.

The tenant is not some hero of the working class fighting against the landlords. They're most likely some shithead swindler who would have stolen someone's primary residence if they thought they could get away with it.