Here the 1841 British edition accidentally omitted the adverb from. The subsequent LDS edition (1849) restored the correct reading. Elsewhere the text has numerous instances of “to flee from before someone”, including this one in the previous verse:
There are three possibilities in all; the text has “to flee before someone” (26 times), “to flee from someone” (11 times), and “to flee from before someone” (9 times), as in the following contrastive usage near the beginning of 1 Nephi:
In each case, we follow the earliest textual sources, thus “they did flee from before us” here in Mormon 2:25.
Summary: Maintain the use of both from and before in Mormon 2:25: “they did flee from before us” (the reading of the earliest text).