Genesis 49:6

God forbid, Jacob is saying, that I should share their evil intent or associate myself with their unjust doings. "Because in the heat of their passion they slew people"; their rage turned irrational.… After all, even if Shechem had sinned, there was no need to turn their thirst for blood against everyone. "And in their fury they cut down a bull"; there is reference here to the son of Hamor, calling him a bull because of his hot-blooded maturity.

Then, in a reference to their crimes Jacob applies a curse in the words "Cursed be their rage for its ferocity and their frenzy for its willfulness": this touches on the stratagem they employed in deceiving the inhabitants of Shechem and imposed on them by guile. Their rage was "ferocious," he says, headstrong, irrational. "Their frenzy is cursed for its willfulness." When the Shechemites came to believe they had won great favor with them, then it was that Simeon and Levi vented their baleful frenzy and deployed the tactics of a foe against them. Referring to their exploit as sins, he foretells as well the punishment for it that awaits them: "I will disperse them in Jacob and scatter them in Israel." They will be scattered in all directions so that this very thing will be obvious to everyone, namely, that they had persisted in committing this crime out of bravado.

As for the historical account, it seems that in this passage [Simeon and Levi] are reproached because, through deceit and fraud, they slaughtered Shechem, the son of Hamor, who after sleeping with their sister had tried to associate himself with the family of Israel. They also destroyed the whole people [of Shechem], so that Israel himself, their father, said to them, "You made me odious in this world." Therefore he curses their cruelty and their recklessness and declares he will scatter them amid the people of Israel, and that from them the Levites and the priests who do not have their own inheritance of land will descend.

(Verses 5 and 6) Simeon and Levi, brothers, carried out the wickedness of their own invention. Let not my soul enter into their counsel, nor let my liver be involved in their assembly, because in their anger they killed men, and in their self-will they hamstrung oxen. By necessity, we are compelled to go further and revisit those things which are in disagreement with the Hebrew truth. For it is written there: Simeon and Levi are brothers, weapons of injustice are their tools. In their secret they conspired against me, so that my soul should not come in: and in their meeting they have cast down my glory, I shall never be cast down. For in their anger they slew a man, and in their self-will they undermined a wall. (Gen. XXXIV). However, it signifies that it was not according to their own counsel that Sichem and Emor killed the allied men, but rather against the law of peace and friendship they shed innocent blood, and with a kind of madness and cruelty, they tore down the walls of the city where they were hosted.

Read the gospel and you will find it written down; the scribes were from the tribe of Simeon, and the high priests from that of Levi. Since the decision to arrest the Christ and to execute him was taken in their council, the prophet foreknowing that said, "O my soul, come not into their council!" In this passage he talks about the council where they took their decisions by searching for a reason through which they might accuse Christ, so that "they took counsel together in order to arrest Jesus by stealth and kill him." And Isaiah says the same: "Woe to their soul, for they have devised an evil counsel against themselves, saying, 'Let us bind the just one, for he is burdensome to us.'