Why is or how did David become the reknown the way he did instead of the power hungry guy he actually was?

You could try /r/AcademicBiblical, but be prepared for a lot of Minimalist views (i.e. the view that everything was invented in Babylon for unspecified reasons). It's an area of history which is a little less rigourous than most, with a fashion for Minimalism at the moment. I won't claim to be any better myself.

Assuming for the moment that David existed, you probably realise that even the Biblical account showing the "power hungry guy" has been cleaned up a bit - for instance the way in which David is distanced from Joab's actions (other than in the case of Uriah). Also some different stories about David have been jammed together with inconsistencies ignored - for instance David appears to meet Saul for the first time twice.

Ok, but as you say, the Biblical account does not show a very nice guy. So why is he important? Well he represented the high point of Israel. He had unified both kingdoms and established a first central administration. From there on it was downhill. Solomon over-spent and had to pay off his debts by ceding about 30 villages and their territory. After Solomon, the kingdoms split again. So David's time represented a peak of Israelite control - whether or not you believe that he actually existed.