Is violence a justifiable instrument for political transformation?
Yes, violence is a justifiable instrument for political change. This may not be true in many cases, but it is true in cases where the status quo in the country is excessively oppressive.
In the world today, we can see examples of places where the oppressiveness of the government means that violence is justifiable. One such example was Libya in the time when it was ruled by Muammar Quaddafi. His regime was so oppressive and did so much harm to his people that violence was justifiable. The same can be said of the Saddam Hussein regime in Iraq.
It may be that violence is not justifiable in less extreme cases. However, there are plenty of extreme cases in our world and violence seems justifiable in such situations.
I'm going to say that it depends on what you mean by violence and what your values are regarding it. "Violence" could mean any coercive action, in which case, if you think coercion is sometimes permissible, you'd think violence is OK sometimes. But if you think coercion is never morally permissible, and that "coercion" and "violence" can be the same, then you'd think violence is not morally permissible--ever. Personally, I'm against coercion altogether. So I think violence is not morally permissible in achieving political ends.