God come to earth for destroying adharma but why he is coming again and again isn't one avathar enough for him to destroy adharma? Why he can't destroy adharma within one avathar?
Lord Vishnu is not responsible to increase adharma on earth. We are the people are responsible to increase adharma on earth. So from this we can not blame lord Vishnu to increase adharma. The measurement of increase adharma is dependent on time and place, it varies time to time and place to place. Like Kali Yuga have more adharma than any other Yugas. So kali yuga also have certain limit to increase adharma, if that limit is reached obviously Vishnu will end this game. And coming to your question Lord Vishnu can kill adharma but not humans. If humans are existing, adharma will be there, only measurement is how much adharma is increased is the matter, if that limit is exceeded Lord Vishnu will come and kill adharma. To remove adharma completely is not even possible until and unless humans exist.
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Dharma/adharma are like 2 sides of a coin - similar to light/darkness. one cannot exist without other. something good right now may not be good always, good/bad are different for different people as well. So called dharma over a period of time will change to adharma, and when it reaches its peak and has to be destroyed, god takes a form and come down - this is what is preached in hinduism or at least from my perspective.