"Cursed be their rage, for it is fierce; and their anger, for it is cruel." (Gen. 49:7)
Jacob's blessing to Simeon and Levi sounds more like a curse than a blessing. Still, the Sages noted that Jacob was careful not to curse his sons, but only their anger.
What does it mean to "curse anger"?
Two Types of Angry Individuals
When a wealthy and powerful person is quick to anger, this is a dangerous combination. Such a person feels free express his wrath. He is constantly yelling and screaming at others, allowing his rage get out of control.
A poor person, on the other hand, even if he has a temper, will work hard to hold his anger in check. He relies upon the favors of others, and can ill afford to vent his displeasure on anyone he wants. We may refer to this type of curbed anger as an "anger that is cursed."
Simeon and Levi's Blessing
Jacob did not curse Simeon and Levi, but their rage. He saw that by nature they were quick to anger, and they needed to learn to control this trait.
Therefore Jacob told them, "I will disperse them in Jacob, scatter them in Israel." As the Midrash explains, they will need to wander around, seeking work as tutors and melamdim. As poor and itinerant teachers, dependent upon the largess of others, they will need to learn to keep their anger in check.
(Adapted from Mishlei Yaakov, pp. 105-106)