Adblock Plus, an ad blocker now used on more than 100 million devices, started out as an almost utopian idea a decade ago. Wladimir Palant, a 20-something developer who was annoyed with intrusive ads online, created it as an open-source project and quickly built it up with the help of volunteers who also wanted to make the internet faster, less cluttered and safer from malware.

Mr. Palant wanted publishers to make ads that did not “degrade” internet users with interruption and animation, he wrote on his blog in 2007. As Adblock Plus surged in popularity, he was able to turn it into a full-time job, increasing its staff to three employees in 2011 and housing it under a company named Eyeo GmbH in Germany. Its motto: “We want to make the internet better for everyone. Purging bad ads is a good start.”

But shortly after that, in 2011, Adblock Plus was altered and became a tool that, instead of blocking bad ads, allowed ads it deemed “acceptable” to be seen, often for a price — a controversial move that has positioned it as a gatekeeper between advertisers and its huge user base.

Now, with a staff of about 70, the company has moved even deeper into that business with an automated online advertising service that will allow more websites to place ads deemed “acceptable” in front of Adblock Plus users. It is a coveted group for advertisers and publishers: Users are often relatively young, well educated, tech-savvy and hard to reach. But the introduction of the service last week ignited a backlash among consumers, who accused Adblock Plus of veering from its purpose.