Why did the detective help drop the charges against the cult leader in Sinner season 2?

It seemed to me she was just as evil as the previous leader.

This seems very debatable to me. We don't see Vera Walker engaging or promoting any violence. She just seems to be a worrying mother and a bit of a control freak.

The only thing "evil" she may have done was poisoning the previous leader, Lionel Jeffries, "the Beacon", and even this is only implied, but never confirmed.

Besides that, everything that we see about Mosswood looks way better with her on charge. No more violence, no more forced abortions, and no more weird parties at midnight with rich folks attending.

With her as leader the place seems to be a hippy commune rather than the increasingly disturbing sex cult that was before.

And it's far beyond any doubt that she loved Julian and that she genuinely cared about him. The last episode made that clear.

why the detective helped get the charges against Vera Walker dropped

Honestly, I don't know, but I think it was because empathy and that "guilt link" that Ambrose shared with Julian, and not wanting to make things harsher for him, getting his mother imprisoned. After all she surrendered, she was not really evil, and in the end no harm was done.

Plus... let's not forget that she gave Ambrose a bit of the rough treatment he likes. Maybe he wants more of that in the future. With Ambrose, I wouldn't completely rule out this.