Why did the Torah see fit to add "Ki Geirim Heyisem ... "?
: 'Do not remark on the blemish in somebody else if you possess the same blemish!'
Refer to Shemos, 22:20:1:1, 22:20:1:2.
In other words, 'Love the Ger and don't aggrieve him!' (See Sifsei Chachamim).