Rashi: One that all his matters are rebellion and refusal, he constantly seeks evil.

R. Yonah: Ish Ra is one whose Zadon overpowers in his heart. When they rebuke him, he stiffens his neck and persists to go in evil ways to rebel against the one who rebukes and defeat him. There is no hope that rebuke will help him. Rather, a cruel angel will be sent from Shamayim to rebuke him.

Orchos Tzadikim Sha'ar ha'Achzariyus DH ha'Achzariyus: This shows the evil of cruelty - his payment is a cruel angel sent against him!

Malbim: A Kesil is Ach Meri. He intentionally rebels, and via rebellion he seeks evil. Desire urges him to intentionally rebel against Chachmah. He seeks evil even if a cruel angel is sent against him. No hitting will help him. If he did not rebel intentionally, Ge'arah should suffice for him. Really, he understands, just he seeks evil via rebellion.