Here the 1874 RLDS edition changed the past-tense conditional if-statement to a present-tense one by omitting the modal verb shouldst and changing would to will. This appears to be a conscious decision since both auxiliary verbs were altered. Nonetheless, the conditional wouldst in the following clause was maintained in the 1874 edition. The 1908 RLDS edition restored the original reading to the RLDS text.
Summary: Maintain the original past-tense conditional modals shouldst and would in Alma 20:18 (“if thou shouldst slay thy son … his blood would cry from the ground to the Lord his God”).