Rainbow Dash isn’t looking jealous here, she’s considering worse case scenarios and how she’ll have to do something very ‘unfriendly’ to keep her friends alive. A pure evil Celestia and Luna are among those worst case scenarios.

There’s a reason why RD chose the Rogue class in D&D.

Yknow the argument that anyone with super speed and some degree of super strength can defeat almost anyone if they are brutal and efficient enough?

Yeah.

When Evil Celestia and Luna start chatting up Twilight, the second the two stop looking at RD, RD quickly flies into a giant arc behind the two alicorns, gaining more and more speed. Rarity notices but decides to keep quiet. Twilight ends up shocking Evil Celestia, which annoys Evil Celestia enough that she decides to throw Twilight behind her back with telekinesis into a mountain.

Mid-throw, RD slams right into Evil Celestia’s neck at break neck speed, saving Twilight. Normal Celestia, who was exhausted from a prior event and who is feeling major injuries her alternate is feeling right now, falls to floor, basically saying the impact nearly broke her neck. RD speeds after to staggering Evil Celestia to kill her, but Good Sombra puts up a barrier up between the two, saying that one or two strikes like that might kill Evil Celestia, but another strike so suddenly with definitely kill the real Celestia.

Luna did NOT seem happy after Celestia decided for both od them that they were going to arrange Twilight’s ascension and their retirement shortly after Sombra returned the first time.