I'm sad to say that I can very clearly see a path by which someone in Nicola's position could choose not to carry on living.
Picture it; feel it, if you will -
Your mood's a bit low. Try to self medicate with alcohol. Get too much into the habit of doing so. Alcohol makes the low mood worse and stops antidepressants working, and interferes with your sleep so you're constantly knackered.
Maybe this is causing arguments at home. Your friends all think you have a lovely life, and you can't admit to anyone else that you've got a drink problem because you feel so ashamed, and that makes you feel even worse.
Maybe your partner issues some sort of ultimatum; "get yourself sorted out, you've got to stop drinking". But it isn't that easy. You're so enmeshed in the worsening feelings of low self-worth, shame, low mood etc etc.
Things get out of hand one night. Maybe one of you raises your hand to the other. Maybe you say you've had enough, you're going to end it all. The kids are crying. A glass gets smashed. The police and HCPs are called. Everyone calms down BUT because of this, social services are now involved.
You "know" that your children are going to be taken into care (you're probably wrong but you convince yourself otherwise). You "know" that you're wrecking their lives. You "know" that the kids and your partner would be better off without you.
You've also not slept well for AGES - hot sweats, alcohol and depression all interfering with sleep patterns, maybe the dog barks at night or the children are restless. How can you think straight?
So, in the befuddlement of alcohol, mental illhealth and lack of sleep, you decide to free your family from the burden of having you around.
I don't have any knowledge of Nicola's family and I am NOT saying that this is just what happened. But I can all too easily see this as a possible pathway. I'm surprised that so many people don't seem to see the same. I would like to be wrong.
(Oh, and regardless if there's any truth in the above re NB, if anyone else recognises themselves on a similar spiral, please PLEASE talk to someone about it. Children are NEVER better off without their mums. People do care. Help can be had).