Starro the Conqueror

Of The Suicide Squad. While Starro is a mind-controlling parasitic kaiju, but also a brightly colored starfish with rounded almost cuddly features and a puppy-dog eye, it doesn't become an active threat until the final quarter of the film, but the very reason Task Force X goes on the mission in the first place is because of Project Starfish, and while characters like the Thinker and Presidente Luna/Suarez attempt to exploit it for their own ends, all of them ultimately pay for it in the end.

Example of:
Big Bad