Rashi: They have venom to kill people, like snakes.

Radak: Their evil is recognized on their faces. They cast poison from their mouths and faces when they get angry, like a snake injects venom. It says "Chamas" even though it is not Samuch, like "Chalas Matzah Achas" (Vayikra 8:26) and similar verses.

Rashi: When a snake ages, it becomes deaf in one ear, and seals the other with earth, so it will not hear what a charmer whispers to it to stop it from damaging.

Radak: Because he compares them to a snake, he said that they are like the harshest kind - a deaf Pesen that blocks its ear. It makes itself as if it is deaf, so it will not hear what they whisper to it [to stop it from damaging].

Malbim: A snake bites a person without benefit, only due to hatred put between it and man - "Hu Yeshufcha Rosh v'Atah Teshufenu Ekev" (Bereishis 3:15). It bites man amidst fear lest man kill it. So is the anger of these Ba'alei Leshon ha'Ra. It is as if they fear me, lest I do evil to them, therefore they slander me. However, a snake hears Lachash and does not damage; they whisper to it not to fear man, for man does not seek its harm, and then it does not bite. After I 'whispered' to these people, i.e. showed them via cutting Sha'ul's robe that I do not seek his harm, they should have hearkened! However, they blocked their ears, like a snake that covers its ear so it will not hear Lachash.