Pilate MINGLED the blood of the Galileans worshippers with their sacrifices

From Egypt, Joseph took Mary and Jesus to return to Judea in Mt 2:

22 But when he heard that Archelaus was reigning over Judea in place of his father Herod, he was afraid to go there, and being warned in a dream he withdrew to the district of Galilee.

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During Archelaus' rule, he had to deal with worshippers at Passover on temple ground for their seditious activities, Josephus' "Antiquities of the Jews", Book 17, Chapter 9, Section 3:

He sent a regiment of armed men, and with them a captain of a thousand, to suppress the violent efforts of the seditious; before the whole multitude should be infected with the like madness: and gave them this charge, that if they found any much more openly seditious than others, and more busy in tumultuous practices, they should bring them to him. But those that were seditious on account of those teachers of the law, irritated the people by the noise and clamour they used to encourage the people in their designs. So they made an assault upon the soldiers; and came up to them, and stoned the greatest part of them: although some of them ran away wounded, and their captain among them. And when they had thus done, they returned to the sacrifices which were already in their hands.

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The Jews won the initial fight. They went on to continue to perform their animal sacrifices in the temple.

Now Archelaus thought there was no way to preserve the intire government, but by cutting off those who made this attempt upon it. So he sent out the whole army upon them, and sent the horsemen to prevent those that had their tents without the temple, from assisting those that were within the temple, and to kill such as ran away from the footmen, when they thought themselves out of danger: which horsemen slew three thousand men: while the rest went to the neighbouring mountains.

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In the final fight, Archelaus killed 3000 worshippers around the temple area.

Emperor Augustus didn't like this massacre and dismissed Archelaus and replaced him with a Roman prefect/procurator.

By the 3rd year of Jesus' ministry, Pilate was the Roman prefect of Judea.

ESV, Lk 13:

1 There were some present at that very time who told [Jesus] about the Galileans whose blood Pilate had mingled with their sacrifices.

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The Jews informed Jesus of a similar incident ordered, this time, by Pilate. The numbers involved were likely a lot lower than Archelaus' massacre.

NIV:

Now there were some present at that time who told Jesus about the Galileans whose blood [accusative, direct object] Pilate had mixed with their sacrifices [genitive].

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These Galileans did something that Rome and Pilate didn't like. The soldiers attacked them at the temple area while they had the animal sacrifices with them. Their human blood was spilled onto the animals. I don't think the human blood and the animal blood were carefully combined.