What if the person being listened to in condition #1 was a trusted employee at the company who is interested in hiring you?

My Grandpa was a smart man, so I’m sure he would have had some specific advice for this situation. The best I can add is that my Grandpa was a big believer in meeting people and forming his own opinion.

In my experience most humans fall into category number one.

Really? I've had almost the exact opposite experience.

I've only encountered a handful of situations where someone went out their way to bad mouth someone else to a potential employer. Out of those, only one or two instances swayed the decision maker.

To OP - it's certainly possible that you were indeed bad mouthed. However, the chance of one disgruntled person's opinion affecting someone's hiring decision is pretty slim.

I honestly wouldn't worry about it. Anyone who takes a strangers word about someone else over forming their own opinion is probably not someone you want to work for anyway.