I really hate the destruction or attempted destruction of art in order to bring awareness to a social cause. I get in this case the painting is highly protected, but there have been plenty of other instances where this has happened to other art where that wasn't the case.
Not only are you a self-entitled piece of shit for tying to destroy something that is on display for public enjoyment, but you are virtually guaranteeing that anybody who didn't already agree with you won't take you seriously because you are acting like such a piece of shit.
Seriously, there are a lot of legitimate reasons for civil disobedience and public protest. This is not the way to go about that, and if you think it is then fuck you in particular.
Edit: I didn't think this was going to be such a divisive issue. After some further research I am retracting my earlier statement about other art being damaged in these protests because I don't see much evidence for that after all. It seems like these protestors are often targeting art they know will get maximum media exposure without causing lasting damage.
HOWEVER, I still think this type of action is counterproductive when you are trying to, hopefully, win over people that either do not support or are not aware of your message. Collective action is an effective means to make change in society. I am, again, not disputing that. I just think that if the goal is to gain broad support for your cause you need to choose targets that are more representative of that cause; rather than art, which does get media exposure, but which ultimately serves to obfuscate or overshadow the true purpose behind your protest. Being savvy about your target audience goes further and deeper into the social zeitgeist than simply getting headlines for being angsty.