It is not about Osama walking around or Osama greeting the fishes. For me personally, it is just making Al-Quaida methods legal by just letting somebody else be doing it.
Like they did, the USA never had a proper trial for Bin Laden, in which he could have had the chance to explain himself or defend his innocence (As if he could really expect it, but justice doesn't come by revenge). He was never accused of anything specific. He was just the CEO of the Al-Quaida franchise, which made him the enemy number one. There was just a Fatwa by the US government, that Bin Laden is the worst of them all.
Bin Laden likely never planned any attack himself, but he had maybe his fingers on the money. Bin Laden was the unifying factor in Al-Quaida, with his doctrines being accepted by most fractions.
The danger is not in treating Bin laden the same way as the good US guy who killed a few people with his car. The danger lies in treating Bin Laden different in front of the law. Once you start having special categories for criminals, with some having to expect less justice than others, only because you feel like they don't deserve any rights, you are on a slippery slope, because exactly such rights for criminals make the difference between a Rechtsstaat and a badly organized tyranny like China.
You can complain a lot about treating a criminal better than how he treated his victims. Sure. That is the reason why he is a criminal and not an over-ambitious law enforcement officer. Also he is only treated better until the court defined of what he was actually guilty and how his punishment should be. Just because somebody said he is guilty, that doesn't make him that. The process defines if he is guilty or not, until there is no doubt left why he got punished that way. In the ideal case at least.
Bin Laden was NEVER responsible for his actions. Nor do we know what his actions have actually been. Death is no punishment, if hell is on Earth.