Stephanie is a danger to the Logan family. She has angered many people. And since Stephanie is so disliked by everyone, many people had a reason to shoot her. Here is my list of suspects:
Donna is a younger and sneakier version of Brooke. When Donna first came to town, she set her eyes on Ridge Forrester only because he was in love with her sister, Brooke. Donna had a lighthearted romance with Ridge, but much to her dismay, Ridge remembered his love for Brooke and told Donna to hit the bricks. Donna then moved on to Thorne Forrester. Thorne was in a vulnerable position since the death of wife, Darla. Donna took full advantage of the situation and lured Thorne into nearly marrying her. Well, nothing gets this girl down. She then became fixated on Eric. They both became intimate, and now Eric is enamored with Donna. Donna is prime suspect #1 because she realizes that Eric is strike three for her - after her failed relationships with Ridge and Thorne - and perhaps her last hope to secure her place in the Forrester family.
Stephen Logan Sr.
He was a terrible father to Brooke and her siblings. Brooke often references Stephen as one of the reasons for her failed relationships. Stephen has been in and out of the picture during the past few years, trying to win Brooke's forgiveness. Brooke’s rape and the thought of almost losing her children were too much for Stephen to handle. With Stephanie out of the way, Stephen may see this as redemption for his precious Brooke.
Stephen (Storm) Logan Jr.
His nickname is Storm, because he held such anger for his absentee father. Storm has a sordid history with the Forrester family. He dated Taylor briefly before she left Storm for Ridge Forrester. Storm's mother had an affair with Eric Forrester, which Storm frowned upon. Stephanie Forrester has made life for his sister, Brooke, unbearable. With years of anger towards the Forresters, Storm wanted revenge.
There is a reason why I did not mention Brooke as one of the prime suspects. Stephanie has tormented Brooke for years: handing Brooke a gun and telling her to end it all, inadvertently helping Brooke's attacker, attempting to take all of Brooke's children, etc. As a result, Brooke is the obvious choice for a suspect. If she wanted to shoot Stephanie, she would have done it by now.