People (not just HN commenters) hate paypal because of a bad reputation that it earned over a long period of time for having horrible customer service, making opaque, terrible decisions that affect people financially, having a terrible UI, and maintaining a monopoly position for largely legislative and legacy reasons. It's fairly common for people to have a personal story about how paypal screwed them, rather than hearsay.

Honestly, I think they've improved vastly on most of this since, but their bad reputation is entirely deserved.