And we are witnesses (kai hēmeis martureōs). Compare “ye yourselves know” (Act 10:37). 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 (Act 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 hōn (which) is attracted into the genitive case to agree with the antecedent pantōn (all), a common enough idiom.
Whom also they slew (hon kai aneilan). Second aorist active indicative of anaireō with a as often in Acts (Act 2:23; Act 5:30). But note kai (also) in the old MSS., not in the Textus Receptus. They “also” slew him, went that far, “this crowning atrocity” (Vincent), kai could here be “even.”
Hanging him on a tree (kremasantes epi xulou). This same expression used by Peter in Act 5:30 which see note for discussion.