This has probably come up before. If so I apologise, because the last topic related to this hasn't been active for a month or two. Unless I missed something.
Anyway! I'd like to make this OP short and to the point. While I could point a finger at what the so called extremists are doing right now(ISIS, unrest in Germany, Norway, Sweden etc) I'd rather target the religion itself. Or rather its trilogy: the Quran, the Sira and the Hadith.
Now, I'm not claiming to be an expert in the field. Perhaps the stuff I'm saying is out of ignorance. But I believe there is one hard fact everybody will agree upon with me. And that is that in order to be a Muslim you must recognize Allah as your only God(and Allah basically means God in Arabic anyway), and Muhammad as his last prophet.
This is a very hefty topic so I'd like to begin with a fairly short argument as to why I think Islam is not a religion of peace. If we look at some of the major religions, many of them have a founding figure, fictional or not, that sets an example how one should act if one is to practice their faith. This is one aspect of these religions that I think makes them more successful compared to purely polytheistic religions of the past. In case of Christianity it is Jesus. In case of Buddhism it is Buddha. And in case of Islam it is said in the Quran that the followers are needed to emulate Muhammed's behavior. In each case the founder, or the head figure that is described in the scriptures sets an example for his followers.
Jesus taught humility and forgiveness. In all of his journeys Jesus said only a few controversial things. Claiming to be a son of God could be considered one. And then he said something about conquest once but hey... in general Jesus leaves the impression of a narcissistic but harmless hippie(from an atheist's perspective). He didn't exactly hurt anybody on purpose.
Buddha was fairly similar in his ways. Although Buddhism is vastly different from the Abrahamic religions, Buddha's strategy was similar to Jesus'. He traveled from village to village and spoke to those who were willing to listen. He's probably the least controversial figure of all. It is pretty hard to find anything remotely violent in Buddhist teachings, the Sutras. Impossible, I'd say.
What about Muhammed though? An overwhelming amount of sources suggest that he was a bloodthirsty warlord who spread his religion through violence and deceit, unlike the other two figures I mentioned before. Here are some of the verses taken from Quran that cannot be associated with a 'religion of peace' on any level:
47:4 So when you meet those who disbelieve [in battle], strike [their] necks until, when you have inflicted slaughter upon them, then secure their bonds, and either [confer] favor afterwards or ransom [them] until the war lays down its burdens. That [is the command]. And if Allah had willed, He could have taken vengeance upon them [Himself], but [He ordered armed struggle] to test some of you by means of others. And those who are killed in the cause of Allah - never will He waste their deeds.
8:12 [Remember] when your Lord inspired to the angels, "I am with you, so strengthen those who have believed. I will cast terror into the hearts of those who disbelieved, so strike [them] upon the necks and strike from them every fingertip."
9:30 The Jews say, "Ezra is the son of Allah "; and the Christians say, "The Messiah is the son of Allah ." That is their statement from their mouths; they imitate the saying of those who disbelieved [before them]. May Allah destroy them; how are they deluded?
2:286 Allah does not charge a soul except [with that within] its capacity. It will have [the consequence of] what [good] it has gained, and it will bear [the consequence of] what [evil] it has earned. "Our Lord, do not impose blame upon us if we have forgotten or erred. Our Lord, and lay not upon us a burden like that which You laid upon those before us. Our Lord, and burden us not with that which we have no ability to bear. And pardon us; and forgive us; and have mercy upon us. You are our protector, so give us victory over the disbelieving people."
4:101 And when you travel throughout the land, there is no blame upon you for shortening the prayer, [especially] if you fear that those who disbelieve may disrupt [or attack] you. Indeed, the disbelievers are ever to you a clear enemy.
48:6 And [that] He may punish the hypocrite men and hypocrite women, and the polytheist men and polytheist women - those who assume about Allah an assumption of evil nature. Upon them is a misfortune of evil nature; and Allah has become angry with them and has cursed them and prepared for them Hell, and evil it is as a destination.
As you can see an alarming number of verses are dedicated to indoctrinating Muslims about Kafirs(unbelievers) and how they should be dealt with. The Quran is very clear about the distinction between Muslims and everybody else. The verses are hateful, and militant, and while some of them might have a bit of wiggle room for interpretation it would be extremely difficult to fit them into the context of an allegedly peaceful religion.
But there are also quite a few peaceful verses that Islamic apologists are so fond of. So the usual excuse I see most often is that well, Muhammad was a man of his time and thus what he did and said could be excused. Surely we wouldnt' do anything of the sort nowadays. To that I have to ask why didn't Jesus ride a dragon to raze Rome to the ground? Surely Jesus was a product of his time too. And so was Buddha. Yet they could still maintain a fairly peaceful message throughout their journey despite the hostile reception they might have received. Not Muhammad though. That guy simply wouldn't turn the other cheek.
It simply doesn't work this way as evident by what is going on today, in the Muslim world. Not to mention if you look at history it is even bloodier. There is no excuse for what Muhammad did, in or out of his historical context. Even if we assume that the verses which preach Jihad are 'outdated' there's obviously an alarming number of Muslims who believe the Jihad verses override the peaceful ones. This conflict will never end simply because Islam is dualistic and rotten at its core. And if you ignore roughly half of what the Quran tells you to do, as a Muslim, can you even be considered a Muslim anymore? That's not how religion works. It's all or nothing.
There's also this alarming habit of pointing at other religions and saying they are just as murderous to somehow diffuse responsibility. It is true that perhaps some of the Crusades(although I personally believe they were launched in defense against the Muslim invaders, unlike what the mainstream sources tell us) were unnecessary and caused many deaths. But we can safely say that these were radical measures and the people who did this were contradicting the peaceful message of the New Testament. But can you say the same about extremist Islamists? Because unlike Jesus, Muhammad conquered with violence too back in his day. So within the context of Islam, the extremists aren't doing anything wrong compared to the crusaders.
In any case, if you think I'm wrong in any of this feel free to correct me. And I'd really like to see a sound argument for Islam that can somehow prove that it could be a religion of peace.http://quran.com/http://www.politicalislam.com/pdf/WebSitePDF/Hadith.pdf