The 1874 RLDS edition omitted the indefinite article a in the phrase “under a consciousness of his own guilt”. The 1908 RLDS edition restored the a to the RLDS text. The Book of Mormon consistently uses the a in the expression “under a consciousness of one’s guilt”:
The critical text will follow the consistent reading with the a.
Summary: Maintain in Alma 14:6 the use of a in “under a consciousness of his own guilt”; all other instances in the text of this expression have the a.