Plot hole: Deviants were created to get rid of dangerous local predators allowing intelligent life to thrive on the planets Arishem 'seeded'. He then created the Eternals to get rid of the Deviants once he realised they eat the species they should have protected. Problem is, it is stated that the Eternals go through their extermination routine multiple times. But the 'mistake' can't be happening all over again in a cycle, and Deviants would ruin a planet if left unattended.
Suggested correction: Arisham made them both part of the cycle to get the planet to be filled with intelligent life. So he always introduces Deviants before Eternals. It's a good way for him to tell the Eternals they are there to protect the intelligent life against the Deviants without knowing their true purpose. To keep them busy.
He refers to the introduction of Eternals as something he did "to correct my mistake." By that definition, he keeps making the same mistake over and over. If he just told them that they have to protect humanity and help their progress, he would obtain the same purpose. After all, for 500 years without Deviants the Eternals stayed put and passive as instructed and didn't create any particular trouble -if they did, they are a limited number he can easily pluck them out of the planet much more easily than a non-specified amount of ever-evolving beasts that he admits are out of his control and can grow much more powerful if they manage to kill Eternals.
He is lying. They are not a mistake, and they are not beyond his control. That's one of the main reasons why they go against Arishem in the first place, because everything is a lie, even after learning their true purpose. The Deviants are there to give the Eternals purpose, helping the intelligent life on the planet flourish, in the most natural way, for reasons we can only guess. The Deviant is upset too, learning the truth that they are only put on the planet for the Eternals to kill.
