It heavily depends on a variety of factors and new considerations:
How rigged is the democracy that is present in a given society? Are there any peaceful loop holes that are reliable enough to create positive change? What is the degree of oppression to necessitate violence? What type(s) of oppression are present?
I think that if a democracy is not able to handle or meet the basic necessities of its citizens, Or lacks basic infrastructure for its citizens, Or is actively committing negative actions against innocent citizens (Such as genocide, enforced slavery, etc) then a controlled form of goal-driven violence is justified, just as long as the violence is funneled for the goal of making conditions better. Violence without a goal, or that ultimately does not help towards a goal to justify itself, is both pointless and unethical.
If a democracy prevents people from having basic rights and/or liberties, then the circumstances that would justify goal-driven violence aren't nearly as urgent, given that the prevention of rights isn't preventing someone from meeting their basic needs and necessities. As long as the democracy isn't severely rigged enough to prevent change democratically or through peaceful protest, and the aforementioned, then Violence can be deemed unnecessary. Violence in this second set of circumstances would most-likely almost never have to considered, and should only ever be considered if absolute stagnation is persistent on a permanent basis.