«فلان لا خير فيه إلا أنه يسيء إلى من أحسن إليه»
There is no goodness in him, except that he mistreats those who have treated him well.
The opening denies all commendable goodness. The particle *illā* then seems to promise an exceptional merit, but it introduces the vice of repaying kindness with harm. Far from restricting the initial condemnation, it reinforces it.
Because the supposed exception is itself blameworthy, the structure functions as emphatic blame through apparent praise.