In their anger they slew a man

Prophecy

While blessing Simeon and Levi, Jacob prophesied about their single murder - Simeon and Levi are brothers; Instruments of cruelty are in their dwelling place. Let not my soul enter their council; Let not my honor be united to their assembly; For in their anger they slew a man, And in their self-will they hamstrung an ox. Cursed be their anger, for it is fierce; And their wrath, for it is cruel! I will divide them in Jacob And scatter them in Israel - Genesis 49:5-7

It appears as Jacob is talking about their ruthless act against Shechem, but in Shechem they killed many, not one person, then why Jacob prophesied about their single murder  - On the third day, when they were sore, two of the sons of Jacob, Simeon and Levi, Dinah's brothers, took their swords and came against the city while it felt secure and killed all the males. They killed Hamor and his son Shechem with the sword and took Dinah out of Shechem's house and went away - Genesis 34:25-27

Fulfilment

Actually this was a prophecy about Jesus Christ crucifixion, and it fulfilled when Levi (high priest Caiaphas) plotted against Jesus Christ and crucified him on the cross -Then those who had seized Jesus led him to Caiaphas the high priest, where the scribes and the elders had gathered. And Peter was following him at a distance, as far as the courtyard of the high priest, and going inside he sat with the guards to see the end. Now the chief priests and the whole council were seeking false testimony against Jesus that they might put him to death - Matthew 26:57-59

with Annas the high priest and Caiaphas and John and Alexander, and all who were of the high-priestly family. And when they had set them in the midst, they inquired, “By what power or by what name did you do this?” Then Peter, filled with the Holy Spirit, said to them, “Rulers of the people and elders, if we are being examined today concerning a good deed done to a crippled man, by what means this man has been healed, let it be known to all of you and to all the people of Israel that by the name of Jesus Christ of Nazareth, whom you crucified, whom God raised from the dead—by him this man is standing before you well - Acts 4:6-10