Laura is actually one of the six women who kept the secret about the night of 3rd January (Juaca, Altagracia, Ricarda, Chichita, Marina, Laura) She also used to be Ernesto's fiancee. And she killed Marina because that woman got pregnant with Ernesto's baby.
*Incase you didn't know, Marina is Altagracia's half sister.
You see, Laura used to be Juan Vicente's lover. While she was with him, he promised that he would let her inherit part of his fortune when he dies and that he would make her the president of Bodegas del Toro. But instead, Juan Vicente leaves everything to his grandaughters, Gloria and Sagrario. (Sagrario is Altagracia's brother's daughter and Ernesto's niece since Bartolome marries Ernesto's sister)
Then not long after that Sagrario was killed by LMdJ. Infact, I'm pretty sure that it was Laura who killed Sagrario. After her death, Sagrario's fortune ws divided among 4 people, Ernesto/ Gloria/ Alirio & Ismael. But 75% of Sagrario's inheritance now belongs to Gloria.
Basically, the answer to the question 'why Laura?', well it's all about the money.
If you're didn't which one is Laura, this is her...