In both PoS and PoW, if the miners as a whole act badly, they will lose their investment. However, with PoW, the underlying coin will survive. This is an important distinction. If a large PoS miner were to successfully act badly, they could potentially enter into derraritive contracts to offset the coin they have "locked up" that is mining. This removes the requirement for a PoS miner to actually invest in their "mining" operation. OTOH, a PoW miner cannot easily hedge their investment if they intend to act badly while mining.