Here Oliver Cowdery later inserted in 𝓟 an ampersand before the conjunction because (the & is supralinearly inserted at the beginning of a line and with heavier ink flow). This correction may have occurred when he was proofing 𝓟 against 𝓞. But it is also possible that this addition represents editing on Oliver’s part. Another editing possibility would have been to insert the and before “they sought”:
Since either reading is theoretically possible here in Alma 14:3, the critical text will accept Oliver’s correction as the reading of the original manuscript.
Summary: Accept Oliver Cowdery’s insertion of and before because as the probable reading of the original manuscript: “and because they had testified so plainly against their wickedness / they sought to put them away privily”.