Musar do Liczb 32:14

And so, with Moses our teacher, may peace be upon him, because he said (Bamidbar 32:14): "a growth of sinful men," there descended from him Shevuel ben Gershom, who ministered to idolatry, as Chazal have said (Bava Kamma 109b, Yerushalmi 9:2). And more than this we find with Eliyahu, who, because he said (I Kings 19:10): "I have been very zealous for the L-rd … for they have forsaken Your covenant, etc.", he was removed because of this from prophecy and the Holy One Blessed be He commanded him to anoint Elisha in his place, as Chazal state (in Yalkut Melachim).

Even though anger is a very bad quality, nevertheless, a man should conduct himself at times with the quality of anger. For example, when he rebukes the wicked, and when he desires to cause a spirit of awe and respect to dwell on the members of his household, and when he wants to obtain respect and even fear from his pupils. Even he who is angry with the wicked, must weigh the extent of his anger, for Moses, our teacher, upon whom is peace, said when he spoke wrathfully against Reuben and Gad : "A breed of sinful men" (Num. 32:14). And, therefore, we find that his descendant (the Levite mentioned in Judges 18:30) became a priest to idols, and this even though Moses was angry because he strove for the glory of God. Everything requires a proper measure in all a man's ways. A man should speak carefully how to do the commandments, whether he rebukes out of anger or rebukes joyously.