It was really all those other events that did it — Dan's gaslighting and manipulation, sure, but also Betty losing full custody of her children, feeling cheated in her divorce settlement, and being so consumed with bitter rage that she was unable to relate to her friends anymore, thereby losing her tether to the real world. Her increasingly unhinged behavior was alarming to the point that seemingly everyone in her life was worried she'd have the urge to do something drastic, but no one thought that she really would.