He was convicted of running a website that let people sell things.
He created a website that facilitated a lot of illegal stuff and got wealthy from it. Now, I happen to think a lot of that stuff shouldn't be illegal but it was and still is and it's not a surprise that the government went after and convicted him, especially with Big Pharma not liking the competition and funding a lot of politicians' campaigns.
Not to forget the whole hiring a hitman to assassinate a thief thing. Perhaps he was egged on by that FBI agent guy, Carl Mark Force or whatever. But still, he could have said "no, that's a dumb idea."
Having said that, I think he can do actual good outside of prison, as something of a celebrity / figurehead.
I don't know how much Trump actually knew about Ulbricht when he made his promise but I wouldn't be surprised to see him slow-walk any pardon.
Usually the POTUS reserves these types of pardons until their final days in office. So when he said he would do it on Day 1, yeah that's a bit sus.