John Calvin Commentary Psalms 26:9

"Gather not my soul with sinners, Nor my life with men of blood;" — Psalms 26:9 (ASV)

Gather not my soul with wicked men. Having now affirmed his innocence, he resorts again to prayer and calls upon God to defend him. At first glance, indeed, it appears strange to pray that God would not involve a righteous man in the same destruction as the wicked. However, God, with fatherly indulgence, allows this freedom in prayer so that His people may, in this way, correct their anxieties and overcome the fears with which they are tempted.

David, when he conceived this supplication, in order to free himself from anxiety and fear, placed before his eyes the righteous judgment of God, to whom nothing is more abhorrent than to mingle good and bad together without distinction. The Hebrew word אספ, asaph, sometimes means to gather together, and sometimes to destroy. In this place, I am of the opinion it means to gather into a heap, as used to be the case in a confused slaughter. This was the objection stated by Abraham:

That be far from You to do after this manner, to slay the righteous with the wicked: and that the righteous should be as the wicked, that be far from You. (Genesis 18:25)

Let us remember, therefore, that these forms of prayer are dictated by the Holy Spirit, so that the faithful may unhesitatingly assure themselves that God still sits in judgment over every person's case, in order to give righteous judgment in the end. In the second clause, instead of the phrase wicked men, he uses bloody men, amplifying what he had said. For although many wicked men do not rush all at once to murder, yet over time they harden themselves to cruelty; nor does Satan allow them to rest until he plunges them into deeds of blood.