When an armed group is formed among an oppressed population in the absence of any democratic mechanisms that establish a popular consensus in favour of armed struggle, the group immediately has two sets of enemies; firstly, the stated enemy who is oppressing the people, and secondly, anyone among the people who disagrees with armed struggle. Targeting this latter enemy is generally logistically easier, safer, and is viewed as essential for gaining societal influence. Critics must be silenced, either through intimidation or elimination. They must be silenced because, by and large, their criticisms cannot be answered. And it is precisely because these criticisms cannot be answered that the group was formed without a poplar consensus; they cannot rationally defend their strategy (if they in fact have a strategy), and therefore cannot build genuine grassroots support for it.
By “genuine grassroots support” I do not mean applause, for they may well receive applause and praise, because oppressed people tend to praise anyone who appears to stand up against their oppressors. But this is merely lip service. Genuine support for a militant group means practical support in the form of cooperation, local grassroots funding, logistical assistance, donations of supplies, protection, concealment, and so on. You can think of the early days of the Irish Republican Army when the overwhelming majority of Irish citizens acted as a comprehensive support network for the Volunteers.
When the justification for forming an armed group is exclusively based on moral or religious rhetoric, i.e., “we must fight for our liberation…we have to stand up for our rights” and so forth; everyone may agree with this in principle, but without an articulated strategy of how and why armed struggle will succeed, genuine popular support will be hard to come by, as well it should be.
When a group among the oppressed decides to take up arms, the consequences for the general population are massive. Guerrilla groups are generally fought indirectly by collective punishment against the people as a whole to undermine any support or potential support upon which the guerrillas may be thought to depend. Predictably, the oppressed become the victims of reprisals in response to the activities of the armed group. The people must, therefore, agree to this. They must be so supportive of armed struggle that they are willing to face intensified persecution, and this willingness must be voluntary, not coerced.
If the armed group does not generate this kind of support organically, they will likely try to oblige the population to support them through intimidation, violence, and fear; and this is the beginning of the end for such groups; you can think of the Tamil Tigers, for example, or indeed, of ISIS. Having established themselves upon abstract moral justifications, they eventually devolve into criminal gangs that operate tyrannically and terrorise their own people. This is almost an inescapable pattern.
Such groups ultimately succeed only in escalating atrocities, intensifying repression, and increasing the misery of their people, usually prolonging violent conflict for years, if not decades. Your goals and your strategy must be realistic and practical, not just morally satisfying.