Why these Servants ganging up on Nero with malicious intent? It's because they all felt that they were screwed over in the Fate/Extra Anime and they're blaming Nero for hogging the spotlight. Tamamo, Berserker and Lancer are upset because their only appearances was just silhouettes and offhand mentions but for the latter two, they're more miffed that they both died offscreen and their Masters were off using their abilities in their stead. As for Assassin, he's more disappointed by the fact he was reduced to a insane monster instead of the honorable warrior that he normally is.

If your curious why Fenrir from TAS is here, it's because he was a Servant of Misao Amari but he appeared in the drama CD under the name Vánagandr and he was of the Rider class. He had ability called Soul Eater that allowed him to eat other Servants to grow stronger, which in a tournament-based Grail War like in Fate/Extra, would make him broken as hell. Too bad that he had a pissant Master and it's a shame that he was unceremoniously killed off without a fight. For Fenrir, this more than insulting and after he's done kicking Nero's ass, he'll ask Lancer for rematch. Hopefully, it will be grand battle to the death.

Fate/EXTRA Encore was a mess and I hope that CCC anime at least becomes bearable.

Fenrir by siba81715928 © Lifewonders Tokyo Afterschool Summoners
Tamamo-no-Mae by ataC2nd © Type Moon Fate/Extra
Cu Chulainn ‎ by Tsubuan
Li Shuwen by sunamin_mmd
Lu Bu by Something Wild
Nero Claudius by Akane