She doesn't consciously decide to kill Frank (and in the end doesn't kill him, but maybe that would've been nicer).
It's just that she's missed two periods, her appetite has bottomed out and her emotions have shot through the roof (and she's proud of the way she hasn't randomly burst into tears around Frank, yet, because neither of them are the best at handling that), and it's only a random talk with Jamia that made her go out and buy the pregnancy test.
And now she's staring at the thin blue line and trying to add up dates, because she can't remember the last time they forgot a condom or she forgot her birth control pill.
Somewhere in the middle of it she starts getting angry. And somewhere in the middle of that, she decides that since Frank hasn't come home yet (probably he's still at the studio, fucking around with his guitars), she'll go find him and give him the news.
Except "giving him the news" is apparently "slamming him against the wall and kneeing him in the balls (and she can see his eyes cross and roll up, it's kind of impressive) and spitting the news in his face before letting him go and stalking back to the car". She's halfway back to the apartment before she realizes what she did, and has to pull over and hyperventilate.
Once back in the apartment, she has all night to wonder if she's finally, after, like, eight years, managed to drive Frank away. (She calls Mikey in a panic around 3 A.M. She's probably lucky he adores her, because she blubbers at him for the first twenty minutes of the call.)
And her streak of not crying around Frank? It's totally fucked when he walks through the door and sees her curled up on the couch. He reacts a lot better than she expected, but mostly because he doesn't try to calm her down, just holds her and says "It's alright, baby, it's alright."