1)
Why should one not believe him?
1.
Malbim: His voice appears to supplicate, but his heart has many abominations.
2.
Malbim (the Nimshal): If he prays at length, do not believe [that his heart is to Hashem]. His heart is full of abomination.
3.
Ohr Yechezkel (Darchei ha'Avodah p.240): One should not judge a Rasha favorably.
