how does napoleon react to the destruction of the windmill? what is his motive?

The destruction of the windmill fits right in with Napoleon's plans; it's a tool of manipulation. Snowball is blamed for the ruination of community project, and Napoleon gets exactly what he wanted, control over the other animals. Complete control. Afterwards, Napoleon supports the rebuilding of the windmill, something he wanted all along but now he has initiated the project; it becomes his.