Like most religion, it is peaceful, but some take it too far, and become violent. This happens not only with islam, but also all other religions. For those who would say that it does not include christianity and there are nkt christian terrorists, look at Planned Parenthood attacks or the recent white nationalist/nazi terrorist attack.
Open up to a random page in the Koran. Start reading. It can really be summed up that easily. I'm only still typing because of the minimum word count requirement. Nineteen words left, now. Fifteen. THE BEST TIME TO WEAR A STRIPED SWEATER IS ALL THE TIME. WE WUZ KANGZ.
Islam, as a religion, contains many violent things that would typically be considered to be horrendous. For example, the Quran supports the beating of women and the killing of people of other religions. This is simply non peaceful behavior.
That being said, most Muslims are still peaceful. Just like with other religions, including Christianity, most Muslims don't follow the Quran to the letter. They instead interpret it in their own way and follow what feels right to them. Islam as a whole, though, is not peaceful.