Yes.If a malnourished twelve year old managed to kill a 30something ex-con who can transform into a Grim, then yes, that would be pretty fucking interesting.

 

Yeah, but if that malnourished twelve year old firmly believes he's killing the man who caused his parents' deaths and the ex-con is wandless and doesn't want to hurt his godson...

Not quite so hard to see happening.

Sirius pulls his punches to avoid hurting Harry and before you know it, Remus and Pettigrew are the last Marauders.

 

If a malnourished twelve year old firmly believes his killing the man who caused his parents' death, he's probably not going to be in the proper state of mind to kill someone - unless said twelve year old is a trained killer. In that fit of anger, he might do something unbelievably retarded like, I dunno, forsake his wand to hamhock the ex-con. Crazy, right?

Just because Sirius isn't gonna kill his godson doesn't mean he's gonna sit there and die, either. He escapes Dementors, outmaneuvers the Ministry and Dumbledore, lives in a cave off rats...and takes it to the jaw while Harry hits him with the falconpunch. Yeah.

 

Thinking about Harry killing Sirius in PoA, I have just one question. How exactly was Harry going to kill him anyway?

 

Who knows? Harry was saying he'd kill Sirius based on emotions, not planning. I doubt [even 'offscreen' so to speak] he ever once gave any thought to the mechanics of it.

Even without training (traditional or otherwise) from qualified wizards, I can't see a smarter Harry not finding a way to prepare for what (even without the prophesy) is sure to be a war in the near future after Trelawney's second prophecy in PoA. The only problem I have with this is figuring out a realistic way to approach gaining Harry some skill as a wizard. Sure, he'd want to study a lot more and learn quite a bit (if he could find a way to pull it off) during the summers, but how would he go about getting a more focussed and more advanced education while at Hogwarts?

 

Apparently, Hermione had absolutely no problem doing this in canon. Hogwarts' education isn't bad - on the contrary, it's considered the best in Europe - Harry and Ron just didn't hardly do shit all to further their abilities as wizards beyond DADA.