Ignorance is Bliss...

I don't know what is.

Yet they're filled with hate.

Ignorance is often times are correlated with a paradoxically higher sense of efficacy. In lay terminology, the less you know and understand, often the more sure of yourself you are. People who are intelligent and who actively seek knowledge will often find themselves feeling that the more they learn, the less they know anything for certain.

Knowledge also burdens the learner with what actions to take about the knowledge they have gained, especially if that knowledge is about injustices occurring and especially if those injustices are systemic and chronic.

People who are unaware of the systemic nature and complexities of unjust systems (ignorant) tend to be more confident in their anecdotal and incomplete assessments and attributions.

I have to say however, that while knowledge can be a burden, it can also be freeing, and while ignorance is correlated with greater self-assurance, as another person who responded said, Trump supporters who appear to be filled with fear and rage, tend to contradict the idea that ignorance is bliss.