You're getting warmer... But the fish was NOT evidence. But there was a trial involved... before this event happened.
Okay, to review the previous hints:
The fish is dead.
The Mafia is involved.
The man knew that the fish meant that he was going to die.
There was a trial.
It was against the Mafia.
Now, if this man is scared of this fish, which has something to do with the Mafia, and this all started because of a trial... What role did the man play in the trial? And what did he do afterwards?