While Islam has historically spread by conquest, it is not specifically stated in the Quran that it should be spread by this method. Like all religions though, Islam does instruct its followers to teach others their beliefs.
The principles of Islam regarding its spread and interactions with other groups is actually very peaceful. Islam teaches its followers to avoid battle whenever possible. The exception to avoiding battle is when self defense is required. Islam also teaches its followers to peacefully coexist with other religious beliefs, especially Christians and Jews.
As for Sharia, while much of it seems horrible to us, it is not unique to Islam. Similar practices, some of which were worse, have been seen in many other religions.
There is no evidence that says that Muslims and Christians cannot coexist peacefully in Europe. If coexistence was taught, most citizens of Europe would have no problem living in a country with people of other religions. While some depict Islam in a fair manner, many people teach Europeans and Americans that all Muslims are extremists, thus causing large amounts of prejudice and tension. If that stereotype was dropped, peace would be more easily achievable.
One thing that is worth noting is that Islam is a religion, and like all religions, can be interpreted differently by many people. The facts that I listed above are not necessarily the views of all Muslims, but I did aimed to find facts that represented the majority of Muslims.
As for the civil war, genocide, and oppression, there are many factors that could be the cause of them. While some is caused by the mixing of cultures, much of the strife throughout history can be traced to other factors such as economical reasons, stress, panic, fear, and hatred, among other reasons. Many societies teach their children to find enjoyment in violence and many children have been exposed to violence. All of these factors contribute to strife that could lead to civil war, genocide, and oppression. Some people also develop a lust for the sense of power that come from killing or oppressing others.