I think you have to want to commit a violent act first.GT and I are more inclined to place the blame squarely on religion's shoulders. Here is my attempt to agree with Cristi (without changing my views of course).
The vast majority of individuals in all populations are non-violent and it's difficult to prod most of them to violence. Violent acts are usually committed by young male losers. There are always people on the margins of crime who judge the costs and benefits of violence to be equal. If murder or rapes rates go up, the extra murders and rapes are likely being committed by those people on the margins, the ones who initially had the greatest propensity to violence. This is probably true no matter what the extra incentive to violence is. Incentives could include rising rates of return to violence, few available women or texts that appear to demand violent acts.
So Cristi is probably right that those who fly planes into building were naturally more inclined to violence than most people, but that doesn’t mean that religious ideology isn’t the thing that pushed them over the edge.
Addendum: There's not my context given for Cristi's quote above. This would have been better
I don't think people commit violence just because their ideolgy says that it's okay to do so. I think you have to want to commit a violent act first.