Typically, groups with political goals turn to violence when they are unable to mobilize significant popular support for their agendas. Because their numbers and power are small, they think that violence can give them greater leverage; they think that terrorism can give them power greater than their numbers. But the usual result of this is that it turns people away from them; people who may otherwise be sympathetic with their goals (even if they did not actively support them), will actually disassociate themselves from those groups, and even condemn them, and whatever they were calling for will be lost because of contempt for their tactics.
If armed struggle is undertaken, it must be undertaken after grassroots support has been cultivated and there is a community-wide consensus that militant action is justified and necessary. While there may be rare exceptions to this, they are exceptions that prove the rule.