Here the 1830 typesetter accidentally replaced to with and. In accord with the reading of 𝓟, the 1908 RLDS edition restored the original to. There is nothing grammatically inappropriate with having an explanative infinitival clause after the verb phrase “about to ”, as in the following example from the same chapter:
In that example, the 1902 LDS edition accidentally replaced the to with and, just as the 1830 edition did here in Mosiah 19:21 (see the discussion under Mosiah 19:6). Of course, there are also examples of “about to ” where and is used instead of to, as in the following example:
We therefore follow the earliest reading in determining whether the following clause is connected by and or to to a preceding “about to ”.
Summary: Restore in Mosiah 19:21 the infinitival to in “to put them to death”, the reading of the earliest textual source (here the printer’s manuscript).