Why did He say "Evil Ami"?
Radak: If they were wise, and recognized the good path, and knew Me, that I bestow good and harm according to their deeds, they would not see the flag and hear the Shofar [of war].
Malbim: They lack Da'as, to understand Hashem and His Mishpatim clearly. An Evil is unsure about everything; it is the opposite of Da'as.
Why does it say "Osi Lo Yada'u"?
Malbim: They lack also understanding to comprehend Hashem via proofs of intellect and investigation.
Why are they called "Banim Sechalim"?
Radak: They are called Bnei Kel - "Banim Atem la'Shem Elokeichem" (Devarim 14:1). These children are foolish, and not wise. I.e. their understanding and Chachmah is not considered understanding and Chachmah, rather, folly, since they did not know Hashem. It says below (8:9) "vi'Dvar Hashem Ma'asu v'Chachmas Meh Lahem", and Yeshayah (2:6) said "uv'Yaldei Nochrim Yaspiku." The complete Chachmah is only Toras Hashem - "Ki Hi Chachmaschem u'Vinaschem l'Einei ha'Amim" (Devarim 4:6). Since they do not know Hashem and do not do His Mitzvos, their Chachmah is schemes and strategy to do evil, not to do good. However, the Torah teaches a person to do what is good and straight.
Malbim: They lack also Chachmah known via tradition. The primary ways of Chachmah are in conduct of the soul, to know good and evil. "They are Chachamim only for evil"!
