Here the 1830 typesetter removed the subordinate conjunction as, thus making the original subordinate clause into a main clause. He also placed a semicolon between that clause and the following main clause (“therefore it was expedient that the curse should fall upon them”). The original construction (“as-clause + therefore + main clause”) is found elsewhere in both the original and the current text; see the discussion and list of examples under Mosiah 23:12. Here in Alma 3:18 the critical text will restore the original as and remove the semicolon that would then lead to premature closure; the expected punctuation is a comma.
Summary: Restore in Alma 3:18 the original subordinate construction with its conjunction as and with no semicolon before the following main clause, thus “nevertheless as they had come out in open rebellion against God / therefore it was expedient that the curse should fall upon them”.