Why did Alison kill Jenna from Pretty Little Liars. I know she did I just don't get why?

Alison DiLaurentis killed Jenna Marshall in "Pretty Little Liars" out of fear and desperation. Jenna was a witness to the events surrounding Alison's secret life and her role in the incidents leading up to her disappearance. Alison believed that Jenna could expose her, which drove her to eliminate the threat. This act was rooted in Alison's need to maintain control over her secrets and protect herself from being discovered.