If Cream left Tails Doll, Amy or Tails will show up to thwart him from killing her upon confronting her in Volcano ruins.Tails Dolls first shows up to rescue Cream from Execun (either from escaping from Exebark, or from an Execun-disguised Vanilla).If the Forrester is already killed earlier, Exeller will kill Amy without problems. The Forrester shows up to save Amy from being killed by Exeller, enabling her to escape his grasp.
He literally can't do anything else because his instinctive bloodlust will push him into another killing spree.
Even when he succeeds in his goal of attaining ultimate power and his revenge, he'll eventually reset the timeline anew out of sheer boredom. Being Evil Sucks: Exeller notes during the worst ending in Whisper of Soul that despite his sadistic pleasure while on a murderous rampage, his inborn psychosis and mastery of time magic means that he has no actual freedom, and his own nature guarantees that he's locked into an eternity of repetitive slaughter.Back from the Dead: Metal Sonic previously thought to be destroyed by Exeller from Spirits of Hell makes an appearance in Knuckles and Amy's duo ending and the True Ending route where Amy personally fights it.His lackey Execun even more so as unlike him, he fully embraces his psychotic behavior who takes pleasure in killing others in brutal ways.He only shows his psychotic side after Laura's death, and even then, he still is shown to deeply care about Alice, as seen in the Crazy Amy ending if Tails tells Cream Robotnik killed Amy's parent on purpose. It also is a downplayed case: because of some Laser-Guided Amnesia, he was mentally stable for a while, his teacher even stating Exeller has a perfect attitude. Justified as he is a Zero-class Highest, a class that are noted to be inherently psychotic, which only boiled over after his and Laura's death. In some cases Exeller will simply kill Amy outright since she's no longer useful for him. Amy if you choose to view her past, being hellbent on killing Robotnik, even killing Knuckles and Tails for not cooperating with her.
Artistic License Physics: Despite being a stuffed toy without an endoskeleton (his only robotic part being his head which serve as his power core), Tails Doll is capable of moving his limbs, and lifting things and people (like rockets and Cream and Tails).