The original text has were here in Mormon 4:12, but the ellipted subject is obviously the singular wickedness, not the nearby plural words (which probably led to the original text having the plural were). The 1920 LDS edition and the 1953 RLDS edition each made the grammatical change to was. The critical text will restore the original were despite its ungrammaticality in standard English. See under subject-verb agreement in volume 3 for examples in the earliest text where subjectverb agreement was based on proximity rather than semantics.
Summary: Restore the original were in Mormon 4:12; although the subject for the verb is the singular wickedness (which occurs considerably earlier in the passage), the nearer plural words led to the use of the plural were in the original text.