If we felt like it—and we do—we could interpret this line to mean that madness is mentally liberating. It frees your mind and allows you to see beyond the humdrum of everyday. Madness gives you divine, godlike insight that in some ways frees you from the boring, linear perceptions of life. Or maybe it just makes you too crazy to recognize that your ideas are just as boring as everybody else's.
'Tis the Majority In this, as all, prevail— (4-5)
The poem paints the Majority as a major force of oppression. What's worse—it's a major oppressive force that always wins. (Yeah, that's the worst kind.) People whom the Majority labels as crazy might be free in their minds, but they're never going to escape from the prison of thought set up by the Majority. If this is the case, would it be better for the so-called crazies to just cut their losses and go with the flow?
Demur—you're straightway dangerous— And handled with a Chain— (7-8)
Hard to miss the freedom and confinement in these last two lines. The symbol of a chain is pretty much synonymous with imprisonment, right? Here, the speaker makes the Majority seem pretty hardcore. All a person has to do is slightly disagree, and everybody treats them like they're running around threatening people with a knife. We're left really feeling the injustice of these chains.