Nagini lost her human mind when she became a snake. She was not a human trapped in a snake.

Many criticisms of Fantastic Beasts involve Nagini being a woman, and I have heard the theory that she joined Voldemort because he was the only person she could talk to, but the Nagini we know from the books is just a snake. She lost her human mind when she transformed according to JKR. She probably joined Voldemort because she didn't have the intelligence of a human and he gave her food.

She is a Maledictus. That means someone who carries a blood curse. They appear to be like an Animagus, which is something in the Wizarding World which means someone can change at will and transform back into the creature with which they have the most affinity with, I would say, but a Maledictus is something very different. Slowly, over time, they are turning into a creature and they can't stop it. They can't turn back. They will become a beast. With everything that implies, they will lose themselves. They'll be alive but in beast form whereas an Animagus retains their human brain. - J. K. Rowling about Nagini in the CoG Blu-ray Special Feature Credence, Nagini and the Circus Arcanus.