Tafsir of Al-Qasas 28:8

The Lam in { ليكون } (that he might be): This is the Lam of purpose (lam kay), which signifies causality, just as when you say: "I came to you li-tukrimani (so that you may honor me)."

However, the meaning of causality here is metaphorical, not literal. Their motive for picking him up was not that he would become an enemy and a source of grief for them; rather, it was affection and the desire to adopt him.

Yet, because this outcome (being an enemy and a source of grief) was the result and fruit of their picking him up, it is likened to the motive for which an agent performs an action. This is similar to "honor" being the result of "coming," or "good manners" being the fruit of "discipline" in your saying: "I disciplined him li-yata'addab (so that he might be well-mannered)."

The Analysis: The rule for this Lam is the same as the rule for the word "lion" (asad); it is used metaphorically for something that resembles causality, just as "lion" is used for someone who resembles a lion.

{ وحزنا } (and a source of grief): It is also recited as hazanan. These are two linguistic variants, like 'adam and 'udm.

{ كانوا خاطئين } (they were sinners): They were wrong in everything; thus, their error in raising their own enemy was not something strange for them. Alternatively, it means they were guilty and criminal, so Allah punished them by having them raise their enemy—the very cause of their destruction—with their own hands.

It is also recited as khatin, which is a simplification of khati'in, or meaning those who missed the mark of correctness and fell into error.