Asking when it is worth using violence to achieve something pretty much means the same as asking "when is it morally acceptable?". It's not a very useful question in my opinion. Imagine if we went through a severe famine, and a large portion of the population couldn't get enough food to survive. Then of course people will start using violence, and they are not going to give a damn about whether violence is morally acceptable. The only thing they're caring about is finding a way to stop their hunger pangs.
That was an extreme example, but it serves to show the idea behind it, which is fairly basic. If one's situation is good, then they will probably see violence as unnecessary and immoral. But the more unstable the situation, the more violence will be seen as a useful course of action. Those who stick hard to morality will rapidly find themselves in a disadvantage to those who do not. And even when everyone is well off, there's always individuals who want more, and are willing to compete with others to get it. The use of competition to gain an advantage is simply a natural result of game theory applied to evolutionary processes. And of course, in the world we live in, not everyone is well off, and resources are neither infinite nor equally distributed. Hence, conflict, to some degree, is always present.
