The Muslim conquest of the Indian subcontinent led to widespread carnage because Muslims regarded the Hindus as infidels and therefore slaughtered and converted millions of Hindus.
Will Durant argued in his 1935 book "The Story of Civilization: Our Oriental Heritage" (page 459):
The Mohammedan conquest of India is probably the bloodiest story in history. The Islamic historians and scholars have recorded with great glee and pride the slaughters of Hindus, forced conversions, abduction of Hindu women and children to slave markets and the destruction of temples carried out by the warriors of Islam during 800 AD to 1700 AD. Millions of Hindus were converted to Islam by sword during this period.
When Muhammad Kasim invaded Sind in 711 AD, Buddhism had no resistance to offer to their fire and steel. The rosary could not be a match for the sword and the terms Love and Peace had no meaning to them. They carried fire and sword wherever they went and obliterated all that came their way. Muhammad triumphantly marched into the country, conquering Debal, Sehwan, Nerun, Brahmanadabad, Alor and Multan one after the other in quick succession, and in less than a year and a half, the far-flung Hindu kingdom was crushed, the great civilization fell back and Sind entered the darkest period of its history. There was a fearful outbreak of religious bigotry in several places and temples were wantonly desecrated. At Debal, the Nairun and Aror temples were demolished and converted into mosques.[Resistors] were put to death and women made captives. The Jizya was exacted with special care.[Hindus] were required to feed Muslim travellers for three days and three nights.
Pradyumna Prasad Karan further describes Mahmud's invasion as one in which he put "thousands of Hindu's to the sword" and made a pastime of "raising pyramids of the skulls of the infidels".
Ala-ul-Din once asked his Qazi, what was the Mohammadan law prescribed for the Hindus. The Qazi replied, "Hindus are like the mud; if silver is demanded from them, they must with the greatest humility offer gold. If a Mohammadan desire to spit into a Hindu%u2019s mouth, the Hindu should open it wide for the purpose. God created the Hindus to be slaves of the Mohammadans. The Prophet hath ordained that, if the Hindus do not accept Islam, they should be imprisoned, tortured, finally put to death, and their property confiscated."
Sayad Mohammad Latif writes in his history of the Punjab, Great jealousy and hatred existed those days between the Hindus and Mohammadans and the whole non-Muslim population was subject to persecution by the Mohammadan rulers.
