Kevin Bacon alone makes this an 8/10 for me. He plays crazed animal-man like nobody's business! He's so good that at times you feel like you're watching a real story unfold.
Christian Slater, well... is Christian Slater. There's a scene where Slater confronts his older brother who is also a lawyer. The brother looks the part - older than 24, has a bookshelf full of reading material, has a nice desk. But when Slater walks in, the brother's like "holy crap, who let this punk kid in my office? Greta, call security! This teenager stole someone's suit and is interrupting my meeting!" Still, he's not *so* distracting that the movie fails. I think the director was smart enough to work with the hand he was dealt by not focusing more on Slater. There are no late-night office scenes with him pouring through law journals. No solo walks through the park thinking out loud... he's just not a great actor.
But I've wildly digressed. Murder in the First is a solid courtroom drama, and that's not easy to do pull off. The genre is like the ill-fated "romantic comedy" that only really succeeds once every 5 years. This one wins because Alcatraz itself is a character in the film. Not just Gary Oldman as the tempermental associate warden, but the halls, island, buildings, families that live on the island -- lump it together and call it the best supporting actor.