Holy books contain so much violence because they were written by primitive peoples in primitive times in which violence was a routine aspect of life and an accepted means of realizing power and justice. The attitudes of their authors is thus reflected in their writings.
Insofar as these scriptures can be used to support the use of violence in the cause of religion, they do contribute to religious violence. Because people revere the texts, they often regard them as an authority uncritically, and thus follow leaders who purport to find justification in their holy texts for whatever violence they wish to promote. Or people who wish to resort to violence themselves will find whatever support they can in their holy texts, believing that will justify them. The world would certainly be a better place if everyone approached their holy texts more critically and questioned their authority or the authority of those who claim to interpret them, and placed compassion before anger when deciding what is right.
The world would also be a better place if such scriptures had actually been enlightened to begin with. Had they been inspired by a wise and compassionate deity, they would contain clear and consistent denunciations of religious and other unjust violence, and would anticipate and thus speak to those who would use these texts to justify violence, explaining what they are doing wrong and what they should do instead. That holy books never do this is among the many evidences naturalists find that such texts were authored by ignorant and fallible men and are not in fact holy at all.
The worst thing these books do is portray God as condemning unbelievers to cruel and hideous eternal torments. If God thinks unbelievers are fit for eternal misery and degradation, the believer can have no reason to think any more highly of unbelievers (since they cannot imagine themselves being more compassionate than their own God). This nourishes an attitude in which the believer concludes it is morally acceptable to kill or torture or abuse unbelievers, or dehumanize them or treat them inequitably. For God thinks unbelievers are so despicable as to deserve such terrible things, it must be okay with his subjects to think the same. Naturalists conclude that better-written holy books would have condemned such a notion of hell as barbaric and immoral. That instead holy texts support such hells as supremely moral and just is to naturalists another sign of the barbarism and ignorance of their authors.