[ahimel]

Don't be a jerk about it. Don't walk out on them without any notice, or tell everybody you interview with that you need a new job because this company sucks.

And if you've done everything you can to make the transition between you and someone else smooth for them, don't feel guilty. Unless you live in Japan or some place where employees are expected to work for life at the same company, the legal firm isn't expecting that. And if you do live someplace where you'll be expected to work for the same firm forever, talk to your manager NOW. Let them know that this is not what you want, and that you'll be needing other work.

But few places do that anymore. In which case, they weren't expecting you to stay forever, so don't feel guilty about not doing it. Just follow the procedures they ask you to - whether that's 2 weeks' notice, letter of resignation, or what have you - and you'll be fine. They have those procedures because they expect people to leave, after all.