We are now living in the age called Kali-yuga. At the end of this age, after tens of thousands of years, Lord Vishnu will appear in His Kalki form, as a warrior riding on a horse, to destroy all the evil people in the world.
He will protect the good people in this way.
At that time, there will mostly be only wicked people in the world and because they will not follow the Lord's teachings, the Lord will have to come to kill them Himself.
He will destroy all those who do not believe in Him and who do not follow His teachings.