Why Does God Have to Destroy the Earth ?

For the same reason he had to kill every living thing that didn't make it on the big boat He had Noah build.
Same old story, but this time it will be by fire.

The first time is was because of man's wickedness. 

Then the Lord saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually.
The Lord was sorry that He had made man on the earth, and He was grieved in His heart.
The Lord said, “I will blot out man whom I have created from the face of the land, from man to animals to creeping things and to birds of the sky; for I am sorry that I have made them.”
But Noah found favor in the eyes of the Lord. 

Genesis 6

This second time, again, because of mans wickedness

'People are dying': violence forces aid workers out of Central African Republic

Threats, looting and violence prompt withdrawal of aid organisations at height of malaria season, denying access to healthcare and leaving thousands at risk

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Working together to saturate the earth
with life ending evil

The end stage rebellion against God,
It begins with homosexuality and progresses to ;

Obama Service Award Recipient Now Researching ‘Butt-Fisting’

Sendler added that his research of "butt-fisting" and bestiality could have impact on a "global level." 

(he's right about that. I might add; 'could have a burning impact on a global level')

Sendler now oversees a group of research students at the University of Lublin Medical School in Poland, where he is studying zoophilia—people who claim to be sexually attracted to animals—and people who have been "butt-fisted by  human sexual partner."

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It's all going to burn, everything on this wicked rebel planet in rebellion against the Creator!

The present world today is too filthy, corrupted and perverted so God will have to burn it  and all the wicked ones out of existence and start over with a good and clean earth.  

But do not let this one fact escape your notice, beloved, that with the Lord one day is like a thousand years, and a thousand years like one day. The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance.

A New Heaven and Earth

But the day of the Lord will come like a thief, in which the heavens will pass away with a roar and the elements will be destroyed with intense heat, and the earth and its works will be burned up.

11 Since all these things are to be destroyed in this way, what sort of people ought you to be in holy conduct and godliness, looking for and hastening the coming of the day of God, because of which the heavens will be destroyed by burning, and the elements will melt with intense heat! 

But according to His promise we are looking for new heavens and a new earth, in which righteousness dwells.

2 Peter 3