It’s not about it being an easier or quicker death, it’s about it being easier and quicker for the perpetrator. It’s much easier to pull a trigger than to stab someone. She had the same opportunity for both, but the gun was easier.
There’s also a good chance she thought she could play it off as an accident. Obviously that won’t be the case with all the witness statements, but it would have been much harder to claim a fatal knife wound was an accident, and also less likely that an accidental knife wound would be fatal.