Many, possibly a majority of muslims are not violent. But those that are take their justification and their example from the Koran itself. Anyone who has actually read the koran will know that it is a deeply troubling document, because of its violent teachings that are interspersed throughout its pages. Violent speech is never more than a few pages away.
For example when muslims refer to jews as 'apes and pigs' they are only quoting a hadith.
When Hamas in its charter refers to a rock telling muslims that jews are hiding behind it and that muslims should come and kill the jews, Hamas is getting it from a hadith:
"The Day of Resurrection will not arrive until the Moslems make war against the Jews and kill them, and until a Jew hiding behind a rock and tree, and the rock and tree will say: 'Oh Moslem, oh servant of Allah, there is a Jew behind me, come and kill him!'" (Sahih Bukhari 004.52.176)
"Believers, take neither Jews nor Christians for your friends." (Surah 5:51)
"The Christians say: The Christ is the son of Allah; these are the words of their mouths; they imitate the saying of those who disbelieved before; may Allah destroy them." (Sura 9:30)
"Prophet make war on the unbelievers and the hypocrites and deal rigorously with them. Hell shall be their home." (Surah 9:73)
Quran (3:56) - "As to those who reject faith, I will punish them with terrible agony in this world and in the Hereafter, nor will they have anyone to help."
A Ugandan evangelical pastor Umar Mulinde was attacked on Christmas Eve by muslims with acid who were shouting 'allahu akbar'. The pastor's crime apart from having converted from christianity to islam was that he had started supporting Israel.
Mulinde has suffered severe burns and eye damage. He was brought to Sheba Medical Center in Israel where surgeons are treating him free.
This is not the first time that Israeli surgeons have treated Ugandans as this picture from a Ugandan daily shows:
One must not paint all muslims as being violent but on the other hand it would be nice to hear more about muslim clerics condemning violence.
As far as I can see condemnations of wanton violence are the exception whilst extremist clerics preaching hatred and violence abound.
Hate mongering islamic preachers give violent muslims all the justification they need to go on their violent rampages.