> The purpose of a justice system is to keep us safe. Someone who hires a hitman should be punished, so that future people don't do it. Whether they succeed is irrelevant.
One small step further: someone who plans to hire a hitman should be punished, so that future people don't even think about it. Whether they manage to hire one is irrelevant.
Now you're punishing a thought crime and equating that with successful murder even.
Merely thinking about hiring a hitman is bad, yes, and should be punished, yes.
But I would not equate it with hiring one. That's because the expected damage from planning to hire a hitman is smaller than that from actually hiring one. A year of punishment hurts the criminal the same regardless of which of those two scenarios holds, but the social benefit of punishing them is less. Therefore we should punish them less. (Unless we don't care about the welfare of someone who's been convicted at all. I think that's a sick position to hold -- even setting aside the possibility of false positives -- but it has its adherents.)
A real eye-opener for me regarding these questions of weighing the costs and benefits (i.e. the economics) of punishment was Thomas Friedman's book Law's Order. Until I read that, I thought the whole question was just a political fight. And while it certainly is a political fight, there's a much more objective kind of analysis available -- in fact, it's the norm in economics.
One small step further: someone who plans to hire a hitman should be punished, so that future people don't even think about it. Whether they manage to hire one is irrelevant.
Now you're punishing a thought crime and equating that with successful murder even.