When a player is killed in Bang! can they still talk?
The rules don't explicitly cover this, so we must use our own judgement.

When a player dies, they reveal their role and their cards. The dead player has no more hidden information. Therefore, I see no reason why they should be required to remain silent. Additionally, one could argue that they aren't even playing the game any longer, and therefore can't possibly be bound by its rules. (In before someone comments that some Bang! character I don't know about can revive.)

If your play group really has an issue with dead players influencing opinions after death, then introduce a house rule. Dead players can't speak (at least about the game).