فَالْتَقَطَهُ آلُ فِرْعَوْنَ لِيَكُونَ لَهُمْ عَدُوًّا وَحَزَنًا
Pharaoh's household picked him up, with the outcome that he would become their enemy and a cause of grief.
The lām of purpose normally connects an action with its intended goal. Here the household did not adopt Moses in order to make him their enemy. Enmity and grief were the actual consequence.
Rhetorical analysis relates the unavoidable outcome to an intended goal, then explains the special use of the lām through that conceptual relation.