Even by the nightmarish standards of the empire she oversees, Susan Wojcicki, the chief executive officer of YouTube, has had a dreadful start to 2019. During a single week in February, BuzzFeed reported that her company was running advertisements alongside anti-vaccine content; there was a nationwide panic over the platform abetting child suicide; and a viral video showed how pedophiles were flourishing on the site.

And then there was the bestiality.

Thumbnail images implying human-animal sex were discovered next to children’s videos, and after an uproar, Ms. Wojcicki was obliged to call a staff meeting to address it. The topic, she allowed during a recent interview, was one “I never thought I would be handling.”

Ms. Wojcicki, 50, was sitting in a conference room at YouTube headquarters in suburban San Bruno, Calif., wearing jeans, a cardigan and an expression of pure stoicism. In an industry that celebrates eccentricity, Ms. Wojcicki presents as exceedingly normal, bordering on boring, even as elements of her digital realm burst into the real world in forms that are increasingly grotesque and sometimes dangerous.

Several months earlier, not far from where we sat, Ms. Wojcicki had spent 40 minutes hiding from an active shooter: a YouTube user furious about new ad standards, who shot three employees before taking her own life. Ms. Wojcicki said she knew that her policy changes could “upset some people,” but “seeing that someone could be so angry that they would come here is really hard.”

Her tenure as C.E.O. wasn’t supposed to be dominated by pedophilia and attempted mass murder. When she got the job, in 2014, Ms. Wojcicki was hailed straightforwardly as the most powerful woman in advertising, someone who’d helped turn on the cash spigots in her time at Google and would presumably repeat the trick at YouTube. In the five years since, Ms. Wojcicki has introduced new forms of ads as well as subscription offerings for music, original content and the cord-cutting service YouTube TV. But somewhere along the line, her job became less about growth and more about toxic containment.

Political figures and tech luminaries alike are castigating YouTube for not doing enough to rein in the crooks, crackpots, racists, Russian agents and charlatans who call the platform home. New horrors are ceaseless — last month, just before a terrorist in New Zealand massacred 50 people, he urged people to subscribe to a popular YouTuber — and reinforce the view that the platform is corroding society.