I think it is survival instinct. SI is always triggered when you know that you are going to die soon. The method doesn't matter. SI is still triggered even with peaceful methods. There are people who have methods that are the equivalent of pushing a button - ( N, Nitrogen, Gun ), and they still can't make themselves go through with it. If it was just a matter of having a quick and painless method, they would already have went through with it since they don't have anything to be afraid of.
SI just causes people to make excuses because a part of them doesn't want to die. If 95 percent of you wants to die and 5 percent of you still wants to live, your SI will stop you every time. If someone becomes desperate enough and wants to die 100 percent, they will use any method available to them at that point. A good example is hanging. Many people would be afraid to hang themself because it is a terrible way to die. But according to statistics, hanging is number one suicide method in the world, because it's accessible to everyone. If someone reaches the point where they very desperate to die to die, they will use a less peaceful method if it is the only method available.