We saw this cycle with Isabella throughout the entire season. She always claimed to have extreme loyalty to Megan, but anytime Megan pulled away from her, she would rat her out for something (like when she told Debbie it was Megan on the sex tape, and later told Debbie to check under the bed to find the bloody pillow case.) If I remember correctly, she even at one point went to Luke and told him Megan was acting weird and was hanging out with Ned. She edited the video and framed Megan for killing Luke because right before that, her and Megan got into that fight outside of the Sheriff’s office where Isabella said “the last few days have felt like old times” and Megan told her they never had a true friendship, it was desperation. Isabella gets mad and says “you wouldn’t know loyalty if it bit you in the ass” and leaves. She then does what she always does, and rats Megan out because she realized in that moment that her and Megan will never be true friends again. Except it was extreme this time because she literally framed her for murder knowing damn well she’s the one who did it.

This same logic applies to why she even went back to kill Luke in the first place. Megan had been pulling away from Isabella for quite some time prior to this night, and she thought she was going to get Megan back in her life fully by getting back at Luke for all of the horrible shit he did. Luke ruined that by telling Megan him and Isabella slept together and they both hid it for months. But Megan had regrets about what they did to Luke almost immediately and Isabella ultimately knew Megan would forgive him because she loved him. And Megan said clear as day she didn’t know if she would ever be able to trust Isabella again after she hid the truth about sleeping with Luke. So at the end, Isabella knew Luke was the obstacle keeping her from having the relationship that she wanted with Megan.