Belief in yourself and your cause is irrelevant. Most people have that regardless of what they do.
Doesn’t mean they are evil, just that they’re paid enough to set aside any moral concerns they have.
1. If you disagree with the base assumptions of copyright, you are going to misunderstand the law and fail your LSAT/bar exam/etc. You will be blinded by "it's just to keep big companies afloat" to notice the actual rules of things like fair use.
2. If you get a law license but disagree with the base assumptions of the law, you are going to be at a financial disadvantage by refusing to represent clients whose politics disagrees with yours.[0]
3. If you choose to represent copyright defendants, you can only do so to the extent that there is a legal argument that they can make. It is illegal for a lawyer to aid a client in the commission of a crime... and if your legal career is based around the idea that something shouldn't be a crime, then you will need to hold your tongue and pick your battles a lot.
4. If you somehow jump through those hoops and competently represent clients... you probably have sacrificed on your "abolish copyright" ideology a fair bit.
[0] Remember that at this point you are probably deep in student debt - your legal career is predicated on you paying back your loans by charging your clients lots of money.
I used to work as a software developer for a banking startup, and a lot of emphasis was placed on things like KYC, AML and "compliance", whatever that is.
Fundamentally, I hated all those things with passion (including how they force everyone to use a bank to transact - and here I am working for a bank), because I don't believe they any institution or government should be forcibly gatekeeping and spying upon anyone's transactions.
It's really hard to work at a place that contradicts your own convictions. When I was fired, it was a relief.
Maybe a bit of a truism, but it is a popular quote for a reason