If voting is violence, is could be considered “self-defense” for you to shoot up the entire polling center. Would that massacre be “self-defense”?

Of course not because that response would be completely out of proportion. An individual voter is only responsible for a tiny assault on my freedom. Only the sum of all voters is adding up to something concrete.

I will try one last example. If I pull the trigger of a gun, that is in itself not a violent action. But I know that a complex set of chemical and physical processes will lead to the planned damage.
Paying an assassin in itself is not violent, but I know that it will have the same outcome through a set of complex interactions. My intentions are the same, I only try to keep my hands clean.

I dont think that there is a fundamental difference if i use a gun, a killing robot or an assassin. I wanted a violation of someone else's rights and I have acted in a way to make that happen.

I understand that causal definitions are a lot more messy and may lead to conflicts in their interpretation, but I think that the world is complex and ignoring causality oversimplifies it. In wars, we know the real criminals are not the soldiers. They are just victims of their ignorance. Many of them are likely good people that would have been alright in other circumstances. The true evil are the people that plan wars for their own profit and lust for power. A causal analysis will reveal them as the true criminals.

But in the end I think we are really just fighting over words. I think that in most real world situations we would conclude more or less the same actions as moral/immoral.