Previous posts of others covered much.
Just wanted to point out.

DobbyElfLord Azkaban fic "Harry Potter Mercenary" premise is exactly that Harry killing Draco.

IMO, Harry is too valuable, I can see Snape going ballistic over Harry killing his godson (is that canon or fanon?), I think there is a fic somewhere covering Snape trying to hunt down Potter and kill him.
In addition it might be the opening Scrimgoer looked for in order to get his paws over Harry.

 

Pure fanon.

 

Regardless of how Dumbledore or the Ministry would respond to this murder taking place, how would the rest of Harry's environment take it?

In Harry's fourth year, when he was a Triwizard Champion, he was abandoned by most of his friends, even though what he did/didn't do was not that bad, and something that others might consider admirable. Now imagine the reaction if word were to spread that Harry is responsible for Draco's murder. No matter how you look at it, it would have a massive impact on the side that supports Dumbledore/Harry's side. Voldemort already built a sizeable army with a decidely fucked up reputation, and I think Harry being guilty of murder would win him points in the face of the public.
Nevermind how his friends would see him. I have serious doubts that Hermoine and Ron would still be as supportive if Harry pulled something like this.