I hate to type this, I really do, but I think she is capable of murder.
When EJ trashed the apartment and cut up the clothes, the one thing I never saw mentioned was any injury to herself, I'm sure we would have heard about blood or the need for stitches if there was any.
She was not in blind rage, she was in a calculated rage. That, IMO, is the kind of person capable of planned murder. A person in a blind rage, cutting up a hard to cut item like clothing, even with scissors is bound to be slicing and end up cutting themselves. A person in a calculated rage will be careful to keep themselves safe, and do the maximum damage.
On the other side, my personal belief is that she did not kill him. She would not have confessed if she had, IMO. She would have just kept running and that text would never have been sent. I believe that was diversion, plain and simple. She's smart enough to know that prosecuting without a body is hard and even harder in waters as muddy as these have become.
ETA: If she did kill him it is my belief that it was an accident of some sort, and she sent the text to Logan in a half out of her mind state. I don't imagine her being rational if she drugged him and he died. I don't believe she liked Gabe, but when faced with your own deceased child, there would have to be some reaction even if it's only panic.