Since you're telling the story in first person, and the MC does something without understanding why, you could lampshade it. That is, after the fact, your MC could be commenting
I don't know why I did it
Or something similar.
What you're doing here, is you're acknowledging to the reader that the MC is acting seemingly "out of character" - not in the way the reader would have expected them to act. You're letting the reader know that this is intended, not a "mistake" on your part, and that an explanation will be given later. If you think about it, we're quite used to this in literature: something happens, later we find out why.
Taking this route, you don't have to inform the reader of something your first person narrator is unaware of - the reader can come to the realisation together with the character.
Of course, talking of motivation, it is also a good idea to give some hint of what might be affecting the MC. If the enemy's story is similar to the MC's, the reader can draw the parallels without being explicitly told this is what affected the MC. If the enemy reminds the MC of someone, you can describe the two in similar fashion, so the reader too might be reminded of the same character. Etc.