The original manuscript is not extant here. The printer’s manuscript reads thing at the end of the line. Oliver Cowdery ended the final g with a downward swirl that could mistakenly be interpreted as an s, which is apparently what the 1830 compositor did when he set the type from 𝓟 and replaced the singular thing with the plural things. The use of the singular is perfectly correct here: the text is referring to how the Lamanites had been killing their defenseless fellow Lamanites.
Elsewhere in the text we have two occurrences of the same basic phraseology, with the singular thing in “the thing which X has/had done”:
And in both these cases a single continuous act is involved: in the first case, the rebellion of Laman and Lemuel and others on the open sea; and in the second, refusing to help the poor petitioner.
These examples do not mean that there are no examples of “the things which X has/had done”. In contrast to the three original examples with the singular thing, there are two with the plural things:
These contrasting examples simply mean that in each case we follow the earliest textual sources in determining whether the text should read thing or things. The critical text will therefore restore the original thing in Alma 24:24.
There is one more example that shows the 1830 typesetter replacing thing with the plural things:
Summary: Restore the singular thing in Alma 24:24 since the Lamanites are repenting of their single collective act of murdering the defenseless converted Lamanites.