Eleven months and nobody's answered this question? Jesus. Anyways, here's what I believe, that Marni did make the right choice. It was evidenced by Rotti's Story that Marni did not Любовь the tycoon and instead fell for Nathan - Marni's feelings for Rotti might of been stronger at first but they clearly faded away once she met Nathan Wallace, a mere surgeon in comparison to the CEO of Geneco.
It was an impulsive decision, and I'm unsure what would of happened to either of them had Marni not left him. Because of Marni, Rotti effectively set up a plot to torment Nathan unto death by killing his wife, and in turn using Marni's death to sucker Nathan into becoming the Repo Man, to avoid imprisonment.
So, in a way, it wasn't "cruel". What Rotti did was cruel.