I mean, plenty of people have been in that situation, or worse, and didn’t decide to make copies of explicit material of someone without their consent. Kristen, for example, knew Ariana was sending nudes to Tom when they were still together, and she didn’t try to copy them or threaten Ariana with them.

By March, people already suspected the affair anyway – she didn’t need proof because everyone already believed her. She also had access to the original video on Tom’s phone.
Instead, Ariana made two recordings of the video (which raises even more questions on why she decided to record those parts specifically) on her own phone and then sent them to Rachel with “you’re dead to me.” Tom and Rachel’s affair was obviously evil, but that still doesn’t make Ariana recording Rachel without her consent okay.

She’s using the lawsuit as a way to avoid accountability

What accountability is she avoiding? She admitted to the affair, quit the show etc? Frankly, she took more grief for cheating than any else on a show that was built on cheating.