A.T. Robertson Commentary Acts 10:39

"And we are witnesses of all things which he did both in the country of the Jews, and in Jerusalem; whom also they slew, hanging him on a tree." — Acts 10:39 (ASV)

And we are witnesses (κα ημεις μαρτυρες). Compare "ye yourselves know" (verse 37). Peter thus appeals to what the audience know and to what the disciples know. He made the same claim about personal witnesses of the Resurrection of Jesus at Pentecost (2:32). Here Peter affirms full knowledge of the work of Jesus in Judea (for whole country including Galilee and Perea) and Jerusalem (given mainly in John's Gospel). In the Greek ων (which) is attracted into the genitive case to agree with the antecedent παντων (all), a common enough idiom.

Whom also they slew (ον κα ανειλαν). Second aorist active indicative of αναιρεω with α as often in Acts (2:23; 5:30). But note κα (also) in the old MSS., not in the Textus Receptus. They "also" slew him, went that far, "this crowning atrocity" (Vincent), κα could here be "even."

Hanging him on a tree (κρεμασαντες επ ξυλου). This same expression used by Peter in 5:30 which see for discussion.