Because Shadow took everything away from them/him and they/he want revenge.
1) Shadow sealed Mephiles the Dark into the Scepter of Darkness, and helped Sonic, Silver, and Elise to stop Solaris.
2) While Shadow killed the Jackal Squad making Infinite the only survivor left.
Nebula reaction of Sonic and Fiona Fox (Tails' former/ex girlfriend) dating, and falling in love together causing to hurt and break Tails Prower's heart on purpose for some reason (from Marvel Cinematic Universe, and Archie Sonic comic books, Sonic the Hedgehog series)
(Part 1) Mephiles the Dark, Eclipse the Darkling, and Infinite (former Jackal Squad leader; the Ultimate Mercenary) reaction of Sonic the Hedgehog and Fiona Fox (from Archie Sonic the Hedgehog comic books series)
(Part 1) Mephiles the Dark, Eclipse the Darkling, and Infinite (former Jackal Squad leader; the Ultimate Mercenary) reaction of Rouge the Bat watching and keeping an eye on Shadow the Ultimate Lifeform (from Sonic the Hedgehog series)
Maria Robotnik reaction if Shadow the Ultimate Lifeform killed Cosmo the Seedrian to break Tails Prower's heart for some reason (from Sonic X, Sonic the Hedgehog series)
(Part 5) (alternate version) Shadow the Ultimate Lifeform vs. his new enemy? (from Sonic the Hedgehog series) (with The Divergent Series: Insurgent reference)
Shadow: (looking at his new enemy, feeling shocked, revealing Maria Robotnik with a sad and serious look)
(Part 5) Shadow the Ultimate Lifeform vs. his new enemy? (from Sonic the Hedgehog series) (with The Divergent Series: Insurgent reference)
Shadow: (looking to see himself, feeling shocked)
Another Shadow: (evil smile)
Mephiles the Dark, Eclipse the Darkling, and Infinite (former Jackal Squad leader; the Ultimate Mercenary) reaction of Shadow the Ultimate Lifeform for some reason (from Sonic the Hedgehog series)
Apple Bloom reaction of Silver the Hedgehog (from My Little Pony Friendship is Magic, and Sonic the Hedgehog series)
Plankton reaction of Rouge the Bat kissing Shadow the Ultimate Lifeform (from Spongebob Squarepants, and Sonic the Hedgehog series)