Attacking the messenger gives you away as a logically fallacious debater. You might be right, but as you cannot prove it within bounds of morality, you seek to deceive the audience by poisoning the well. You make your opponent appear to be unworthy of credit by using loaded language against them so your audience subconsciously gives them a black mark, having them biased against them and in your favor. If there are judges who are immune to emotional appeals, or your audience are as observant as they should be, your deciet will be conspicuous, your argument seen fallacious, and your cause, even if true, will be failed.