I don't see it exactly this way. I think the scene when they follow her in the corridor and then in her bedroom are simply threatening and scaring. Maybe some people see that scene as grotesque and laughable. I just see it as a horrifying and ravaging warrant from her inner spirit. Plus, we have the famous gesture the tiny version of them does after they crawl under Diane's door. Which is raising arms in the air as if saying 'Not guilty'. Actually, I think Diane's suicide is caused by a great variety of elements of course, but the strongest one that eventually leads her to the action of commiting suicide is the torture of being unaware of what truly happened to Camilla (yes, she is probably dead, but we don't know that for sure since the hit man is not exactly a professional) and being guilty for what she did to her lover and to herself.

"Did they scoff the whole damn Smörgåsbord?" (Audrey) 

"Gimme a donut!" (Coop)

I agree with you the couple is quite scary and the laughing and mocking has this tone now.

In the end, when they are small and echoed, or large and ebullient, all up in her face, the tone of ridicule has shifted up another few notches. As viewers we are horrified right along with Diane to the point of no return and can justify her drastic self destruction. 

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"Did they scoff the whole damn Smörgåsbord?" (Audrey) 

"Gimme a donut!" (Coop)