@Man_From_India "The first thing I'm going to do is killing your daughter," is certainly not correct. Use the bare infinitive after "do", not the gerund. — Andrew1 hour ago
Using "killing" there doesn't sound "natural" at all, but in conversation you could get away with it, I guess.
I have heard it used by nonnative speakers that way.
I think a nonnative speaker believes that putting -ing on the end of "kill" gives more action to the sentence, but it doesn't grammatically work that way :-)
@Man_From_India I think folks try to match "going". "The first thing I will do is kill your daughter..." probably would be less confusing even though it's almost the same thing. "I'm going to eat dinner, take a bath, and then go to bed." No native speaker would say "I'm going to eating dinner, taking a bath, and then going to bed."
I suppose we could say "I'm going to BE eating dinner, taking a bath and going to sleep." but then I'd be eating and sleeping in the bath....