It really depends on the issue and the circumstances.
So for example, if there are any Zimbabweans left plotting to kill Robert Mugabe, then I would consider them wholly justified because they do not have the luxury of peacefully effecting change.
However, there's still a difficulty as governments that are largely democratic do sometimes ignore their people (and sometimes for good reason). The anti-war demonstration against Iraq in the early 2000s was one of the biggest demonstrations in British political history, and almost certainly represented a huge swathe of public opinion, but changed nothing. However I can't see that the followers of that particular cause would have been justified in turning to violence and can't quite put my finger on why.