I agree heo. It's just one of those moments that even in real life would be a tad blunt ~ or else drawn out and melodramatic.
My interpretation around it (stemming from watching Alice carefully throughout that section to read her body language and expressions) is that Diana really isn't sure she wants to leave. I think part of her knows that she has to get out of the house and relationship at least in the form it's in at that point, but part of her desprately wants Dan to give her SOME kind of response besides the "it's ok...we'll be ok" vibe that he's been giving, conceivably for 16, 17 years. I think that had he made any kind of effort to stop her, she would have changed her mind, but the fact that he just sits there cements for her that she HAS to do it.
In a slight change since the 2econd Stage production, largely I think driven by what she's getting from the other actor to play off of, in the version now, there is the slightest moment of hesitation, especially when Bobby's Dan looks up at Diana for a moment, and there's obviously an emotional tug-of-war going on in Diana's head about leaving right before she walks out. She wants to stay but knows it will kill them both ~ and probably Natalie too.
Brian's Dan, on the other hand, never looked up at her in all (14) the times I saw it at 2econd Stage (his big emotional moment of that came AFTER Diana was gone and Natalie had come home). The moment there was all about Diana looking back at him as she was on the verge of walking out, essentially asking with her eyes, her expression and her body language "DO SOMETHING!!! STOP ME!!!" before visibly sighing and walking out.
She doesn't WANT to leave, but she does want a chance to survive and knows that leaving is the only way she'll have a chance at that point. And I think it's a case of she just feels like it's easier to just say it ~ like ripping the band aid off ~ so she doesn't chicken out. Who knows how many times she's been on the verge of doing this and backed out before she got to the actually picking up the suitcase and leaving moment? Is it an awkward moment? Sure. But it would be in RL as well.
Experience live theater. Experience paintings. Experience books. Live, look and listen like artists! ~ imaginethis
LIVE THAT LESSON!!!!!!