Agreed. Judge Not lest Ye be judged.
Allison has made a lot of bad decisions since her carefully constructed world began systematically turning against her.
How can anyone know that, under stress, they wouldn't make a similarly horrible snap decision? --- even something that, in retrospect, was clearly second degree murder?
Allison actions where shaped by her sheltered life. When you're upper-middle class, the constraints of modern life don't tend to bite with the same harshness and finality as they do for others.
Sheltered Allison didn't really think she was killing her best friend. She was just insanely pissed off, and hey, they were all going to laugh about this later over a bottle of Chardonnay, right?