Stupidity, and laziness which affect most liberals and account for nearly half of the population today are self evident and require no further discussion.
But the psychotics are far more troubling and seem to dominate the more ambitious and motivated liberals who are activists and politicians.
Those most deranged can't get their shit together enough to get into office so they resort to protesting, marching, fighting cops, occupying Wall Street, and in extreme cases shooting up theaters and elementary schools.
There is even a subset psychosis which has come about in the last decade called BDS, or Bush Derangement Syndrome. Read more here or go to Family Security Matters and type "liberal mind set" into the search box.