Well it sounds like she thought that the cancer medications were horrible themselves. I don't get the feeling that she wanted him gone -- she went through the first several phases of cancer treatment and then gave up at the very end.
From JPB's article:
LaBrie, testifying in her own defense, told the jury that she followed the instructions from her son's doctors for the first four phases of treatment but stopped giving her son the medications during the final phase because she "didn't want to make him any sicker."
LaBrie said she told her son's doctor two or three times that she was afraid that "he just had had it."
"He was just not capable of getting through any more chemotherapy," LaBrie said. "I really felt that it could out-villainize the disease — the medicine could — because he was very, very fragile."
She was definitely irresponsible to make that decision unilaterally, though, without speaking to her doctors about it. And the fact that she didn't suggests that she knew they would say she should continue.
It sounds pretty possible that if she'd pushed through he would have survived. That's the key point to me I think. This disease has a good survival rate with medication but not 100%, and she stopped the medications two years into the whole thing. So what were his chances of survival at that
point? If they were still high, I definitely think there should be some sort of consequence, though attempted murder doesn't really fit the bill.
Edit: and if his survival chances were very low at the point that she decided to stop giving him medication, I still think the unilateral decision should get some sort of consequence, but lower yet.