Well , considering that Mister threw her out , this might be considered revenge on him. Just too bad for pet Alice. Pretty rare to see "Death by snu-snu" with an Alice at the receiving end.
Considering that she never asked mister if she could had a partner, specially a stray one, this isn't rightful at all, only a petty revenge of someone broken, ironically, by her own actions.
And later you'll see that this mainly was her broken and thinking that anon would accept her again by having "alice babies".
When you take into consideration the fact that this Alice has been kindly raising Marisa's own child all this time, this act only becomes more heinous.
It's like if you thought you lost your child, only your child was actually adopted by a loving family. Then you went and murdered that family.
there is also the irony that Alice did what anon pretty much wanted marisa to do, in a very easy and simple way, and maybe the realization that ALL the misfortunes she faced could had been prevented if she just acted like Alice, well, it may had broken her for good.
You know like a epical "my god what have i done?" hitting her hard.