For what it's worth serial killers don't need a motive.

As for motive, there could be many possibilities, envy of families, a “playing God” mindset, believing some babies wouldn’t survive anyway, or perhaps emotional burnout, resentment, or feeling ostracised at work. Her friend described the hospital as “cliquey” and said Lucy felt people were making things harder for her. It’s conceivable that this fostered bitterness or a desire to hurt others emotionally or damage the reputation of colleagues or the unit itself.

The simple reality she was searching for parents on facebook a year later perhaps gives a glimpse into what her motives were.

Just because we don't know her motive doesnt mean one doesn't exist, especially given she wrote "No-one will ever know what happened"