God, the Creator, is Most Gracious and Most Merciful and Most Kind and Compassionate: He makes the mother kind to her baby.The reason is simple.
Different interpretations of what God is according to their source material (Torah and writings before it).
That's it.
The God of Islam is the same God of the Jews as it is the same God of Christians. But because all of them think they have the sole answer for how to worship said God, they can't not fight among themselves.
You're all vying for the favor of a God you all agree exists, you just aren't sure about how to deal with him in life.
If you'd all just get over yourselves and let people worship as they wish, then there wouldn't be the fighting and death that your so called God abhors (but really doesn't cause he's one of the worst instigators of murders ever created).
God commands peace and goodness, but people choose the evil and transgression .. but God will not leave the transgressor and will not equal the guilty like the innocent .. this is His law of life and for all people.
Moreover, leaving the transgressor and the wrong-doer to his caprice and desire will corrupt people's life, and God misguides such transgressor and wrong-doer so as he will be a disbeliever.
Now disbelievers will not leave people to worship God, but will always and by all means fight the worshiping of God, and it will be necessary that God averts the disbelievers by the hand of believers.
Quran 22: 40, which means:
(Had it not been for that God restrains people: some men by means of others D ,
cloisters [of Christians, which are on mountains and in the rural areas],
and churches
and prayers [would have been forsaken]
and [the Muslim places of worship: the] mosques – wherein God's name is mentioned much [in the times of the five prayers every day] would have been destroyed E .
But God will surely help those who help His [cause]; surely God is Most Powerful, All-Mighty.)
More explanation is in the link:
http://quran-ayat.com/pret/22.htm#a22_40
quran-ayat.com/pret/22.htm#a22_40
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