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 Sh'mos, 22:20:1:1 & 2
In other words, 'Love the Ger and don't aggrieve him!' See Sifsei Chachamim.