Bring a large pot of heavily salted water to a boil; cook the rotini at a boil until tender yet firm to the bite, about 15 minutes; drain and transfer to a bowl.

Bring marinara sauce to a simmer in a saucepan. Add kimchi, chicken, onion, and red chile paste; stir until completely mixed and heated through, about 5 minutes.

Mix butter into pasta and toss to coat. Pour marinara sauce over pasta and top with scallions.