صورة-1
اتق شر الحليم اذا غضب. It would literally be “be wary of the wickedness of the good-natured person if/when he becomes angry.”
At the moment I can’t think of an equivalent proverb in English, but the idea of “beware of the quiet/nice ones” is a common one, especially when someone commits a crime who you would not expect to commit a crime.
As an aside, it reminds me of the proverb “تحت السواهى دواهي” which is not the same of course, but has a related idea. That one is roughly equivalent to the English proverb “still waters run deep.” If I think of