Why are hacks on smart contracts on smart contracts seen as illegal?
What do you mean by it?
Why stealing btc from someone's private key is seen as malicious?
Don't get me wrong here: but your keys, your crypto. I don't think someone can't say the other way arround, right? Ownership can be proven only by having the private key to your coins.
Because by the definition of "steal" conveys it. Yes you may find other belonging, but it is up to you whether to claim it as yours--since you found it-- or return it to the true owner. In another sense, it's up to your moral standard.
And about smart contracts, the moment someone is using a smart contract he agrees with the code of the smart contract.~
Should hacks on decentralysed networks to be not judged?
They might agree but that doesn't mean they have the comprehension. How many of the smart contract users understand software engineering?
It is actually like a regular internet user, how many people do they read the terms of service and privacy policy, it is almost non-existent. ToS and Privacy policy likely contain legal and technical language, a word that may regular Joe don't grasp fully. And here we are dealing with smart contracts.
Back to the question, if the network is truly decentralized, then the participant is inherently judging.