Let's assert that marketing works.
The money and time and effort we put into marketing goods and services actually works. It gets people to change their minds. It cajoles some people into buying and using and voting for things that they otherwise wouldn't have chosen. (If it doesn't work, save your money).
If it works, then, are you responsible for what happens after that?
If you market cigarettes aggressively, are you responsible for people dying of lung cancer?
I think there are two ways to go here:
1. You're not responsible. The marketer is like a lawyer representing the obviously guilty client. Everyone is entitled to a lawyer, and it's up to the jury to decide. The lawyer's job is to do the best she can, not to decide on the outcome. Market the best you can and let buyers take responsibility.
2. You are responsible. Your insight and effort cause people to change, and without you, that change would never happen.
I'm not sure there's a middle ground. Either we should applaud the folks lobbying on behalf of causes we despise, the pornographers selling products that degrade our society and the politicians spinning and lying to get elected (because all these people are doing is giving us a choice for which we're responsible) or we should take responsibility for stuff we sell.
My take: if you're not proud of it, don't sell it.