In case of Islam and Koran, everything that Christianity stands for, is contradicted by Muhammad in Koran. Unlike the content of scriptures of other religions, Koran redefines the events and facts from Bible almost directly on one-to-one basis. Moreover, the reasons for these contradictions are pretty clear.
For Christians, Jesus is the Messiah, the one who by his reconciling death provided atonement for mankind. Koran says Jesus didn't die but was just a prophet and there was no atonement.
About heaven, Muhammad redefined heaven using five passages of the Koran as something like an enormous bordello in the sky.
Heaven is redefined. The work of Jesus on earth is redefined. Even the very nature of God is recast by Muhammad. So whatever Bible teaches as its core message, has already been changed by Muhammad.
There are many verses, to incite Muslims to violence against various kinds of non-Muslims which again directly contradicts message given by Jesus about loving our neighbor to the extent of praying and loving our enemies.
True such verses are found in OT also, but that was a different age and era altogether. In the late Bronze Age, in the time of Moses, Joshua, and right up until King David, human societies apparently had not developed to the point that you could separate a spiritual, religious leadership on one hand from a secular, political leadership on the other. The change was ordained by God in NT, because human societies had developed to a point where the change could be instituted. Yet we see Muhammad has taken it back by several centuries.
When King David said, "I want to build the temple," God said, "No. You will not build my house. You have much blood on your hands. You have fought many wars in my sight. Your son Solomon is a man of peace. He will build my house."
Now the reasons for these contradictions:
It happened because Muhammad had limited knowledge of the Old Testament and even less of the New Testament. But he was pretending that he had complete knowledge. So he kept getting the stories wrong. In the Koran, Muhammad tells the story of Moses' confrontation with Pharaoh, the Exodus story, 27 times in 89 chapters. There's not one mention of the Passover. He told the story of Gideon's, only it wasn't Gideon, it was King Saul.
Koran places the Samaritans at the time of Moses when such people had not existed at that time.
Another contradiction is claiming Mary, mother of Jesus same as Miriam, sister of Aaron and Moses. These two women lived 1500 years apart. The Koran says.
She brought (the babe) to her people, carrying him (in her arms), They said: “O Mary! Truly a strange thing has thou brought! “O sister of Aaron, thy father was not a man of evil, nor your mother a woman unchaste!” Q. 19:27-28
This is repeated in verse 66:12 where Muhammad calls Mary, daughter of `Imran.
Now, it was not until about 200 years after his death that Muslim leaders learned Latin. Some of them learned Greek. They could read the Vulgate Latin Bible or the Greek New Testament, and they began to see that there was a Passover in the Exodus story and thier prophet didn't mention it. So they had to either admit that he was mistaken, he didn't really know what he was talking about, or the option was to blame the Jews for having tampered with the Scriptures, so that God had to use Muhammad to restore the Old Testament to its original form.