YouTube, Twitter and Facebook have been scrambling overnight and into Friday to remove horrific video footage recorded by one of the participants in the terrorist attacks that killed 49 people in New Zealand. A mounting backlash online suggests that many people don’t think the companies are doing nearly enough.

The mass shooting carried out in two mosques during Friday prayers is the worst ever suffered by the small nation, and its reverberations increased because of the primary attacker’s efforts to promote his hateful agenda using digital means. The unidentified man in his late 20s, who police said has been arrested for murder, used a message board on 8chan to announce his plan to stream the killings live on Facebook.

The 17-minute footage he shot with a helmet camera was viewed by more than 200,000 people, according to multiple press reports. It was also mirrored and circulated across the internet for...