Why does the Almighty create defective human beings? The Qur’ān says at so many places that Allah is merciful and kind. So how can a Merciful God do such seemingly cruel things?
To a similar question Jesus (sws) replied: ‘The Almighty creates the blind so that those blessed with eyes can [really] see’.
One must remember that this life is a trial and a test for us. According to the Qur’ān, Paradise is actually a reward of one’s patience and perseverance in times of trial and test:
And because they were patient and constant, He (-- Allah--) will reward them with a paradise great and garments of silk. (76:12)
Those who are patient shall receive their reward in full, without even being called to account. (39:10)
In other words, it is to be appreciated that people who are born with some handicap are brought in this world according to the general scheme of the Almighty and not because they are being punished for some sin. So just as the Almighty gives wealth and affluence to some and deprives some of even the basic necessities, and just as He makes some human beings very beautiful and others very ugly, on a similar pattern He creates some people defective and others totally normal.
Another thing that needs to be appreciated is that the more a person is deprived of some faculty the more he will become liable to relief. He will be held accountable after being given due allowance of this deprivation.