Peter gotti tried to take out gravano, its one of the reasons hes gonna die in prison, and i expect the same result for anybody else dumb enought to try and whack a govt witness. To reiterate PB, damage is already done after they testify, it serves no purpose.
Of course you try to kill him before he testifies, but that is impossible as the witness would be under protection of the US Marshalls 24x7 until the trial starts.
The purpose of killing rats after the trial is to let them know they can't rat and live like normal afterwards. If a guy can rat and go back to the old neighborhood and live a normal life, then there will be no fear of ratting on the mob anytime a person is arrested.
