Even the Buddha of compassion might have been distressed to be on the receiving end of the diatribes that William Lawrence Cassidy is accused of posting on Twitter.

They certainly rattled Alyce Zeoli, a Buddhist leader based in Maryland. Using an ever-changing series of pseudonyms, the authorities say, Mr. Cassidy published thousands of Twitter posts about Ms. Zeoli. Some were weird horror-movie descriptions of what would befall her; others were more along these lines: “Do the world a favor and go kill yourself. P.S. Have a nice day.”

Those relentless tweets landed Mr. Cassidy in jail on charges of online stalking and placed him at the center of an unusual federal case that asks the question: Is posting a public message on Twitter akin to speaking from an old-fashioned soapbox, or can it also be regarded as a means of direct personal communication, like a letter or phone call?

Twitter posts have fueled defamation suits in civil courts worldwide. But this is a criminal case, invoking a somewhat rarely used law on cyberstalking. And it straddles a new, thin line between online communications that can be upsetting — even frightening — and constitutional safeguards on freedom of expression.