People discuss Jasper being an absolute jerk to Amethyst during Season 3 in the context of Jasper having maladaptive views from the war, or some kind of ruthless pragmatism, but I have another perspective on that.

Amethyst isn’t just a defective Gem born on Earth, she’s also a Crystal Gem despite her origins.

That is to say, she was presumably created during the big push of Gem manufacturing during the war, and her intended purpose was to supplement Homeworld’s forces. Even if she was intended to be a higher-quality soldier than the Beta Quartzes, she was still created for the same purpose as the Gems in Beta. But now, here she is, having joined forces with the same Gems that attacked the newborns at the Prime and Secondary Kindergartens; a despicable, underhanded tactic that was responsible for the loss of countless lives, as well as ruining Jasper’s own life as soon as she popped out of the earth.

The thing is, Jasper has absolutely no idea that Amethyst was born a few thousand years too late and spent her youth alone. She has no idea that the Crystal Gems adopted Amethyst and raised her like their own, possibly out of guilt for what they’d done in Prime and Beta. And she especially has no idea that the senior Crystal Gems never told her anything about her sisters, or what had happened to them, other than that they were Homeworld’s soldiers and thus needed to be stopped for the greater good.

So, to Jasper, Amethyst is despicable not because she’s weak and defective, but because, to Jasper’s perception, she’s a defective Gem who turned traitor and joined with the faction that was responsible for murdering her siblings because she wanted to be on the “winning” side. That line, “You’ll follow anyone who makes you feel like less of a failure,” could apply to what Jasper sees as Amethyst’s motivation: that she joined with Rose, the winner, in order to feel strong and worthy despite being born defective because she supposedly knew that she would not have a place on Homeworld, and she did this presumably knowing full well what Rose had done. It also explains why Jasper has a much less negative view of Peridot, despite Peridot also being defective.

The thing is, Amethyst has absolutely no idea about what happened in Beta. The Crystal Gems never told her about the attack on the defective Gems and the newborns, presumably to avoid upsetting her or out of their own guilt. When the two of them start bonding, Amethyst’s lack of acquaintance with her own history will become extremely obvious to Jasper, and Jasper will realize that Amethyst is basically in the same boat as Peridot in terms of information about the war being withheld from her.

There’s going to be a moment of awkward silence as Jasper realizes what an absolute honking douchebag she has been.