Why did the Torah need to add "Ki Geirim Heyisem ... "?
Rashi 1 : To teach us not to remark on the blemish in somebody else if one possesses the same blemish. 2
Refer to Shemos, 22:20:1:1 & 22:20:1:2.
In other words, 'Love the Ger and don't aggravate him!' (See Sifsei Chachamim).