Nonetheless, despite her initially benevolent nature, Nagini's blood curse eventually turned her into a snake permanetly. Though it is unknown if she became evil as a result of that, or was turned evil as a result of Voldemort turning her into a Horcrux. As a snake, Nagini was completely loyal to Lord Voldemort.

How did Voldemort get Nagini?

During this time, Voldemort killed a witch called Bertha Jorkins, and J. K. Rowling confirmed it was through her murder that he turned Nagini into a Horcrux. But Nagini was reintroduced in Fantastic Beasts: The Crimes of Grindelwald, which complicated the previous assumption that he'd met her in Albania.

Why is Nagini so loyal to Voldemort?

Nagini is loyal to Voldemort because he can speak with snakes. As a Maledictus, there probably isn't much time after the events in The Crimes of Grindelwald before Nagini falls to her blood curse, becoming a snake forever.