If you don't believe in free will, you become fatalist, and that's harmful for decision making. Hard determinists also often think in absurdist way, and losing ability into logic is very dangerous. The popular take how to use free will to punish everyone in most cruel way or how to evade such punishment by pretending to be a piece of inert matter is very dumb if not outright malicious and ignores the whole point that free will is primarily needed for the agent himself for survival and isn't needed for most cruel punishments at all. If you really want to cruel, just be cruel for no reason.