Here in Jacob's last words to his sons, we read of two of them, Simeon and Levi, being singled out as cruel. By their actions, they had proved themselves cruel both to animals and to humans. God warns them through their father Jacob that this trait would likely be passed down to their descendants. Because of it, He would be forced to scatter them throughout the other tribes of Israel, diluting its ill effects within the larger nation.
It is interesting that, despite its inherent leanings toward cruelty, the family of Levi was chosen by God to serve Him in His Tabernacle, Temple, and through the Aaronic priesthood (Numbers 1:50). Perhaps by concentrating the members of this tribe on His work, God transferred their aggressive tendencies to a far better purpose.
Is it also possible that, although God softened him with His Holy Spirit and made him the meekest of men (Numbers 12:3), Moses—a Levite—inherited some of his tribe's proclivity towards cruelty? Did he not murder an Egyptian whom he caught beating one of his fellow-Israelite countrymen (Exodus 2:11-12)? Did not Moses' wife, Zipporah, call him a "bloody husband" or a "bridegroom of blood" (Exodus 4:25-26)? Yes, we know that she says this relative to the circumcision of their son, but was there perhaps more to her outburst than just this?
Later, after Moses had been on Mount Sinai for almost forty days, the Israelites persuaded Aaron to make an idol, the infamous Golden Calf, for them to worship. Seeing the idolatrous rites and perversions happening in the camp, God sent Moses down to deal with the situation. Not only did he break the two tablets on which God had inscribed the Ten Commandments (Exodus 32:19), "he took the calf which they had made, burned it in the fire, and ground it to powder; and he scattered it on the water and made the children of Israel drink it" (verse 20)! Incidentally, he afterward called for all those who were on the Lord's side to go throughout the camp to kill their idolatrous fellow Israelites—and the tribe of Levi rallied to him (verses 25-28)!
It seems that Moses had to keep his anger in check throughout his life. After he had brought Israel to Kadesh, just before the final push into the Promised Land, the children of Israel murmured due to the lack of water. In his impatience, anger, and frustration, Moses struck the rock rather than speaking to it as God had commanded (Numbers 20:7-11). Doing so destroyed his chance to enter the Promised Land with the people.