Interspersed with the questions and answers are snapshots from Mr. Yilmaz’s life, including photographs of a new son, and propaganda for the Internet age, like images of Islamist fighters cuddling cats.

Image A screen shot from an Islamic State fighter's Tumblr. Credit... Israfil Yilmaz, via Tumblr

In a television interview with the Dutch state broadcaster NOS early last year, he said he had no intention of ever returning home to carry out a terrorist attack. “I would just eat,” he said, “maybe some sushi, have some Dr Pepper and give my mother a big, warm hug.”

On Friday, however, in response to a question from The New York Times he defended the attacks in Paris by calling them a fair response to the bombardment of Islamic State positions by the French Air Force.

“Would I make a big deal out of it if the Islamic State declared war on Iceland and Iceland started attacking us?” he asked rhetorically. “Of course not,” he replied. “That’s life in the big city, you can’t just declare war on the Muslims (kill their women and children) and expect to be left alone even though you might reside somewhere far away. That’s something everybody understands right?”

Earlier in the week, he offered a similar defense to other Tumblr users, calling assaults on civilian targets in France by the Islamists “fair game,” as, he said, he had “lost count of the hospitals, markets and mosques bombed by the enemies of Islam.”

Last month, in response to a question from The Times about why he joined the Islamic State after previously keeping his distance, Mr. Yilmaz replied:

Ask yourself which other group is implementing the Shariah as complete as possible? Ask yourself which group is fully taking care of the affairs of the people as complete as possible? No other group but the Islamic State, so me joining the Islamic State was just a matter of time, for they are able to govern the people and implement the Shariah on a large scale — protecting the Muslims, their wealth, health and religion.

While other social networks and messaging services have responded to complaints about Islamic State militants using their platforms by blocking accounts, the jihadists and their sympathizers often seek to evade the barriers by simply setting up new accounts under slightly different names. Mr. Yilmaz set up four accounts on Twitter, using the platform to denounce Western bombing of Islamic State militants, before giving up on that service.

While the fact that his Tumblr account has not been blocked could be an oversight by the company that will soon be corrected, the archive of questions and answers offers a fascinating glimpse into the thinking of a Western-educated jihadist who has been transformed in the past two years from a skeptic of the Islamic State to an active member.

UPDATE: Late Friday, after this column was originally published, Tumblr disabled the jihadist’s account.