Radak: Above He discussed trust, which is good when it is it in Hashem; it depends on the heart's intent. Now it discusses intent of the heart. It has the greatest power of deception. One can appear good in his words and deeds, and his heart is evil.

Malbim: One may ask, we see people who trust in Hashem and do not succeed! There are two answers. (a) Some trust openly, but not in their minds. The trust must be with a full heart, and not with deception. Also refer to 17:11:2:2 and the note there.

Rashi: It is full of pretexts and deception, from all evil.

Malbim: All the evil pictures and bad Midos lurk in the heart, which is the authority in the Nefesh. Evil pictures of every Aveirah come in the heart.

Rashi: It is an expression of illness. Malbim - it is prone to reverse at any moment

Radak: It is pain and breakage, like "u'Che'ev Anush" (Yeshayah 17:11), "Anushah Makasi" (above, 15:18). It is an anthropomorphism to discuss pain of the heart in agony, worry or evil deeds or thoughts. Therefore, it says that it is pained - "Cherpah Shavrah Libi va'Anushah" (Tehilim 69:21).

Rashi: The man thinks that no one knows his intents.

Radak: Hashem says, who knows his evil, other than Hashem?

Malbim: If it seems to you that he trusts in Hashem, you do not know his thoughts.