What were these charcters motives?

Why did the detective tease that that Sean's tape proving his innocence couyld 'get lost' if he realised that he was innocent all along? Also why was the psyciatrist out to get him if he also realised he wasnt the murderer everybody thought he was? Surely realising his mistake from the orignal murders he'd just keep his head down and stay quiet rather than try and frame him for another murder?

What were these charcters motives? Why did they have it in for Sean so bad?