2. Why on earth?

'And war broke out in heaven: Michael and his angels fought
with the dragon; and the dragon and his angels fought, but
they did not prevail, nor was a place found for them in heaven
any longer. So the great dragon was cast out, that serpent of
old, called the Devil and Satan, who deceives the whole world;
he was cast to the earth, and his angels were cast out with him.'

Revelation 12:7-9, NKJV 

Put that against the old Genesis story (chapters 2 and 3),
where a snake tempts the first humans to ‘fall’ (i.e. accept evil
into their mind and nature) and here’s the meaning you get:

Evil began in Lucifer by his own choice, and spread to other spiritual
beings by their choice. The first humans experimented with it against
God’s advice, and brought Earth under the control of evil - Satan
became... ‘the prince of this world’ (John 14:30).

God still works with people who rebel against Satan’s dark kingdom
of evil and ‘sin’. God hopes people will see the results of evil and
choose His leadership, rather than blaming Him for what Satan did.
Jesus told people to pray ‘[May] Your [God’s] will be done on
earth as it is in heaven
’ (Matthew 6.10b, NKJV). Then, when it’s
obvious that Satan’s government is to blame for all earth’s evil, God
will get rid of evil and anyone who stubbornly chooses it.

Meanwhile, evil is not fair. Cancer doesn’t care if you’re Mother
Teresa or an axe-murderer. Rapists can pass on AIDS to their
innocent victims. You are less likely to suffer if you practice safe
sex and don’t share needles,but AIDS (like all evil) hits the best and
the worst of us. Jesus said, ‘Rain falls on everyone, good and bad.’

OK - Let's keep moving....