As she presided over the regular luncheon meetings of the Monterey Bay Colony of Mayflower Descendants, Mary Jane Crawford Holmes represented a lineage stretching proudly back to the original Pilgrims.

She never could have imagined that her family’s landing on Plymouth Rock would extend almost four centuries later to a grandson accused of mass murder at a Batman movie.

That sort of violence couldn’t be more foreign to the genteel gatherings in the Beach House at the Monterey Peninsula Country Cub on idyllic Pebble Beach in California. And Holmes was not just a member of the chapter. She was the governor. Her duties included conducting an annual reading of the Mayflower Compact, marking the day in 1620 that the Pilgrims pledged “to enact, constitute, and frame, such just and equal laws, ordinances, acts, constitutions, and offices, from time to time, as shall be thought most meet and convenient for the general good of the colony; unto which we promise all due submission and obedience.”

By all accounts, the Stanford-educated Holmes had always lived up to the principles of the predecessors in whom she is said to have taken an increasing interest after retiring from a long career as a librarian. She had determined that she was a kind of American aristocrat, genealogically speaking. She was descended not only from the Pilgrims, but also from a member of the Ancient and Honorable Artillery Company of Boston, the first American militia. This was the type of outfit that the Second Amendment of the Constitution refers to as “a well-regulated militia, being necessary to the security of a free state.”

As would be recorded in their obituaries by the Monterey County Herald, her husband, Robert Holmes, had been a career officer in the standing army that was subsequently established to keep the country secure. He fought in the Pacific during World War II and stayed on, serving as a lieutenant colonel in charge of a Nike antiaircraft battery in San Francisco at the time of the Cuban missile crisis. He retired in 1963 and began a second career as a teacher at the private York School in Monterey, where his wife served as a librarian. Both their sons graduated from York: the older James in 1967; Robert Jr. in 1969.

The younger Robert went on to earn a bachelor’s degree in mathematics at Stanford, a master’s in biostatistics at UCLA, and a doctorate in statistics at the University of California at Berkeley in 1981. His doctoral thesis was titled “Contributions to the Theory of Parametric Estimation in Randomly Censored Data.” He subsequently authored studies for the Navy and the Marine Corps on how to forecast personnel changes using something called “tree classifications,” the trees in question being statistical. Eventually, reports say, he signed on as a low-six-figure-a-year senior scientist with FICO, which produces management systems, fraud protection, and credit scores. He married Arlene Holmes, a nurse.

On Dec. 13, 1987, James Holmes was born, apparently named after his uncle, in the way of those genealogically inclined. The latest in the line was 3 when his grandfather, the elder Robert, died. James’s grandmother continued on as governor of the Monterey Bay Colony of Mayflower Descendants, which from all accounts now included a grandson as brilliant as his father and seemingly as law-abiding as the signers of the Mayflower Compact could have hoped.

When Mary Jane died in September 2010, young James had just received an honors degree in neuroscience. He had trouble finding a job, but soon began following his father on the path to a doctorate. He continued to seem as well behaved as his lineage would have told his grandmother to expect.

Accordingly, James had no more difficulty than the most upstanding Pilgrim would in passing the background checks that were all he needed besides money to acquire an assault rifle, a shotgun, and two automatic pistols beginning last May.

His forebear in the Ancient and Honorable Artillery Company of Boston may well have been shocked to see how the constitutional talk of a well-regulated militia had been twisted into keeping gun laws so lax as to threaten the security of a free state. The forebear surely would have been horrified that the alleged perpetrator of the resulting slaughter was of his own blood. His grandmother, at least, was spared ever knowing where her distinguished lineage would lead.

On Monday, Mary Jane Crawford Holmes’s grandson will appear in a Colorado court, charged with the murder of 12 innocents and wounding another 58. Those crimes violate everything enshrined in that Mayflower pact of his predecessors, everything the original truly well-regulated militia fought to defend.