I don't think that Satoshi is mainly aiming for bitcoins usage for illegal things, he just want to build something that is good for all of us and just because it has a good way to transact with its anonymity those people who are involved for illegal activities sees its potential. And let's just not be like them who are using bitcoin for bad things.
Yes, the Bitcoin transaction means it's illegal transaction. People use this bitcoin to play gambling and other gambling activities like trading, mining and investing like this. So all are illegal activities. When this bitcoin become legal currency, then we will get a chance to use this bitcoin for good purpose.