You can call me Jiggly
Very quick "emphasis mine" here, as I'm curious - why have you logged Defeatist as strictly negative, whilst logging Slow Start as double-edged - what sort of distinction can be drawn between the two abilities? At a glance to me, they look similar (conditional weakening), just with different enough conditions that I'd love to know the thought process.1. Purely negative. No clear positives whatsoever outside of giving it to the opponent. This route is more about overcoming the flaws of the stage by excelling elsewhere rather than harnessing it. I'd consider Defeatist and Truant some examples of this.
2. Half bad. These abilities are unfortunate, but they have a positive element that can be used, or a double-edged quality to the negative that can be harnessed in some situations. The negative element of these heavily outweighs the positive. These are flexible as you can choose to ignore the positive and tackle it like a purely negative concept, or fully lean into the positive element and minimize the negative. Id consider Slow Start, Stall, Normalize, and Klutz to be examples of this.
e: As to not clog the thread further, pipotchi quickly pinged me their rationale - Slow Start by definition slows, and being a lower speed in some cases being slower can be beneficial to a Pokémon. That justifies the difference in my head happily :-)
More generally: Trying to define a 'bad ability' in a vacuum is tricky - especially as we're sort of trying to play up the stronger aspects of a 'bad' ability. The closest I can get (and this process is making me understand why physicists have spherical cows...) is "in a vacuum of Pokémon which have access to a highly restrictive movepool consisting of effectless STABs, a bad abiity is one where a Pokémon would rather have a neutral ability like Illuminate over the ability".