It's a common misconception to think that 51% attack gives the attackers some god-level powers over the network. 51% actually only allows attackers to revert transactions by forking the blockchain from the earlier block, they can choose to mine empty blocks to effectively DDOS the network, or they can choose to censor certain addresses by refusing to include their transactions and forking whenever other miners do so. 51% attack can't change the rules of the network or do anything that breaks the rules, period.

So, is it a crime? The censorship attack would be the most unlikely to prosecuted as a crime, the DDOS is generally a crime, but I doubt it would viewed like that by authorities, but the last one, the double spending would have very high chances to being prosecuted if it will result in millions of dollars worth of coins being stolen. Authorities already go after Bitcoin thieves, and double-spending is just a type of robbery.