Rashi: With his speech, a hater hides his identity, lest it be recognized that he is a hater.

Orchos Tzadikim (Sha'ar ha'Sinah DH Amar ha'Chacham): Many people show love with their words, and it is possible that in their hearts, they are absolute haters. One must not believe them.

Malbim: In his external speech he appears to be a lover. Internally he counsels how to trick him and make him fall to the pit (grave).

Malbim (the Nimshal): This is a flatterer - he flatters with his lips, and his heart is far from him. His lips feign that he believes in Hashem and serves him, but there is guile in his heart.