There is no justification for the bedroom tax. It is a tax based on ideological spite rather than on saving money or on fairness. There is an ideological minority in the Tory Party that hates the poor and the bedroom tax is am ideological sop to them.
The fact that the bedroom tax had been inflicted on those who live in private rented accommodation before it was inflicted on people in social accommodation is not an argument in favour of the bedroom tax. It is wrong in both cases and, as the old saying goes - two wrongs do not make a right!
As it happens I have some sympathy with the problem of people living in houses that are too big for them blocking housing. My parents are part of the problem! They live in a five bedroom ex-council house - sold to them by the last Tory Government and taken out of the social housing mix. But if they sell it they couldn't afford a smaller house in the private housing market.
If your lot hadn't destroyed the social housing stock in the first place the problem wouldn't have arisen. The Bedroom Tax is punishing the poor for the effects of the anti poor policies of Thatcherism – that you and yours celebrate.
The sickest thing about your policy is that if my parents hadn't wasted money on oversubscribing their little wealth into buying a council house, which may be eaten up by age care costs, they could be living in a council bungalow with a spare room that their children, grandchildren and great grand children could use on visits. But even that would not be allowed if they were re-housed under the bedroom tax!