I'm sure that some people think Islam is "radical" because of groups termed "radical muslims", in other words terrorist groups made up of muslims. For many people who have no experience with Islam or muslims, it is probably difficult to seperate the religion of Islam from the words and actions of violent political groups who use the rhetoric of radical views of Islam. In the same way, many of these groups believe Christians are evil because of the Crusades, although it is difficult to justify a holy war from the teachings of Jesus.
To many modern Westerners much of traditional religion (of any kind) seems archaic. To many in the Islamic world, the image of America and Europe presented by TV and movies is vulgar and offensive. Of course, movies and TV shows don't really represent Westerners accurately, any more than they do muslims. The reality of human beings is far more complicated than sound bites on the news or the made up world of media.
People who call themselves Christians and advocate war against Islam are overlooking the central teachings of Christianity. The same with Muslims and Jews and any religious group which advocates violence as a means of converting others or imposing their religion on other people. If God gave us the freedom to believe anything we want, how can we ask Him to help us force others to our way of thinking?
Western people think Islam is radical for two reasons.
1) Ignorance. Westerners are ignorant of 'normal' Islamic/Arabic culture and only know the media presentations of angry 'warlike' Islam. They do not know the history of Islam/Arabic culture nor the contributions muslims have made to human advancement. There are many moderate Imams and tolerant muslims, but they are not often seen on the news. 9/11 gave most people the belief that all of Islam wants to destroy America/Democracy and they are very frightened by this idea. Ignorance is a very powerful force.
2) Fundamentalism. Islam is going through a radical/fundamentalist upsurge. There is a struggle within Islam for its future direction. All religions and cultures change over time. At the moment, traditional Islamic religion/society is being challenged by secular western materialism and democracy. This is making some Muslims very uncomfortable and angry. They have become radical fundamentalists. They want to create very strict Islamic societies where a hard-line version of Sharia is enforced by extreme methods. There is a battle for the future of Islam between people who want to turn back the clock to medieval times and people who want to modernise Islam. Moderate muslims must protect their religion from these dangerous violent people.
Islamic culture must modernise some of its views about freedom of speech, women's rights, political representation etc etc. These are basic human rights and it is inevitable that Muslims will want them. During this modernisation there will be struggles and extremists who try to drag Islamic society into a new dark age. If Islam is not careful, there will be many wars between muslims over the future of Islam.