Just some thoughts on the subject...
The question whether an individual is justified to an act of violence against another indivudual brings forth the question, whether people have the right to decide on behalf of other people. For example, if we use death penalty as a means to put away dangerous criminals from our sight, does an individual also have the same right to delete another person? For me it's a question of the right to decide about the life of another person and I don't think I have such a right. Acts of violence always bear the connotations of being either vendictively calculous or acting in the heat of the moment. People perhaps feel that if violence occurs when it's not prememditated and done for selfdence, it can be seen as a justified act. What to do, then, when a person is seriously injured and the guilty party for that act says it was done in the heat of the moment - for selfdence. How can we decide if it's true or not? We can only see the injuries, not what happened.
As for whether a legal body has a better right to use violence than an individual person or a non-governmental organisation: if every individual has the right to conduct any violent act they see as justified, where would this world be going? Terrorist attacks are probably seen as such a case: a specific, non-governmental group has decided to take justice in their own hands. They have their own objectives and rules, the government has little chance of ordering them to stop. They can be silenced either by force or negotiation and the problem of negotiation is that both parties must be able to compromise. They cannot achieve any goals they wish without having to give up most of them. So the easy way is to pick up the guns and grenades and just hope that you have more ammo than they do.