When? Always.
This liberal has no problem casting blame: it’s just that dealing with the consequences of human frailty typically involves a broader and usually different set of considerations. That’s been said above.
I’ll note here though that many liberals have a more expansive sense of responsibility than conservatives do. Or specifically, utilitarianism puts boundless moral demands on the individual, as Left Hand of Dorkness points out. (The liberal philosopher Rawls proposes a moral framework with less demanding social obligations, though it goes far beyond what any existing government provides).
We might cast the conservative / liberal debate in another form: when is the average taxpayer not responsible for the bad behavior of the few? The liberal says, “Decide this question with an empirical and tough-minded focus on the consequences of the policies.” In contrast, some conservatives can’t seem to abide spending taxpayer money on those they deem anecdotally undeserving, though I rarely read of serious and systematic policy recommendations along these lines. [1]
Either these conservatives just like to grumble, or they’re more than willing to use anecdotal or apocryphal outrages to cut benefits for the many. I don’t have a problem with those who prefer a weaker social safety net: that’s a personal preference. I just wish for greater honesty.
[1] Time limits and welfare reform are a possible exception.