SEATTLE — Over months of discussions in online forums earlier this year, Paige Thompson acknowledged the personal challenges in her life: suicidal thoughts, struggles to find employment, and difficulties she had faced since transitioning to a woman years before.

But those who knew her were nonetheless stunned by what came next: the arrest of Ms. Thompson on Monday on charges that she had stolen the personal data of over 100 million Capital One customers.

Ms. Thompson, 33, had spent years lurching between a promising career as a software developer and a life of upheaval that alienated her from her friends. While she at times found community among fellow computer engineers, she on other occasions grew confrontational with them.

“It was just a lifelong thing for her,” said Sarah Stensberg, a former friend. “When she gets in these phases of intensity, she does really stupid things. She’ll push everyone away. She’ll write threatening emails. She’ll post things online about the things she’s doing.”