IMO, if you serve content to the user, you're responsible for making sure you're not serving malware, e.g. by not allowing parties you cannot trust to serve arbitrary code.

And this doesn't apply just to the ad network, it also applies to the publisher. I'd really like the publishers to be held liable for malvertising they serve.

There is nothing inherently hard about serving safe ads, we've gotten pretty good at separating code from content nowadays. There is no reason why a "classic adsense" three-blue-links ad should be able to inject malware. Even images can be served safely. The only reason why this happens is because in pursuit of a few percent more profit (and more tracking) everyone allows everyone to include arbitrary scripts.