Lilac she is first of all a professional hitwoman and loves the money too. I don't think she's done any other senseless murders - the exes make sense as she gets enraged when they refuse to belong to her, there is a reson there even if emotional.
But she saw Nico as an annoying obstacle, same as Gemma saw Eve! She tried scaring him off in Oxford.
She egged Gemma on to seduce Nico, it's only when he said he wasn't in love with Gemma that she decided to kill, I still don't see why. She really does NOT like it that Eve has a husband and only stopped short of killing him as that would upset Eve too much even if NIco left Eve.
It's like she didnt want another wirnes, thought G would be too hysterical to stay shtum - maybe.