The original manuscript does not have the subordinate conjunction that. There is no sign of any insertion at this place in the original manuscript. Oliver Cowdery added the that while producing the printer’s manuscript. The conjunction that is, of course, expected here. For instance, in nearly all cases of “X saw (that) they”, the earliest text has the that (19 times). There is one more case without the that; in this instance, the 1830 typesetter added the that, probably because there was a following conjoined clause that began with that:
(For further discussion of this case, see under Helaman 4:24.) In one of those original cases with the that, the 1840 edition accidentally omitted it:
In this case, the RLDS text has retained the reading without the that. Generally in the Book of Mormon, the subordinate conjunction that is optional after the verb see, although in most instances the earliest text has the that. In each case, the critical text will follow the earliest textual sources in determining whether the that should be there or not. Thus the critical text will accept the reading of 𝓞 without the that here in Alma 27:2. For a general discussion of the optionality of the subordinate conjunction that, see under that in volume 3 .
Summary: Remove the intrusive that after saw in Alma 27:2 (thus “and when they saw they could not seek revenge … ”).