He pressed the battle strongly until the sun set, and Demetrius fell on that day: It is not clear in the Greek text who the referent is for the pronoun He. It could be Demetrius or Alexander. The footnote in Good News Bible for this verse is misleading, since the Greek has only a pronoun as the subject of the first clause of this verse. If translators follow Good News Bible in verse 49, Demetrius is the logical referent for this pronoun. The battle scene in verses 49 and 50 can be coherently translated this way:

• 49 When the armies [or, soldiers] of the two kings fought each other, Alexander’s* army began to retreat. Demetrius** pursued them, and was defeating them. 50 He kept attacking them fiercely until the sun set, but he himself was killed.
* Alexander’s; some manuscripts have Demetrius’.
** Demetrius; some manuscripts have Alexander.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Maccabees. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here.