Why Isobel died? It was her decision or it was because of that Klaus's constrain?

Isobel killed herself beause of she though she was a bad mother and infleance

Isobel killed herself because Klaus compelled her to do so when the job was done.

Like everyone has said, Isobel died because Klaus compelled her to after the job was done. But I think she was glad as she was not the one who handed Elena over to Klaus personally.

Because she was a terrible mother and because Klaus compelled her.

She killed herself because Klaus compelled her to do so, but I think that was the one part of her task she was relieved to complete.

Are you sure that Klaus compelled her to do it ? Because when she is compelled, she's not supposed to know it and she asked Elena to come with her, I think she decided to kill herself, she wasn't compelled. When someone is compelled, he/she isn't aware about the orders he/she just does it, right ?