Mostly because Dubya made the decisions with Cheney deciding for him. Cheney, who stood to make a lot of money if Haliburton made a lot of money.
I can imagine that being the son of a president (especially one as disgraced as Bush Sr. was, to the point of pushing people to vote for Clinton instead) gives you a lot to work for...and I'm willing to bet that Cheney latched onto him for that reason.
Who doesn't want to be more like a relatively successful father/figure?
So with wrong information and, at the behest of Cheney, the US went above and beyond what we should've done and, in the process, pretty much created ISIS. Those deaths are on the hands of the commander in chief at the time, but Cheney was pulling his strings.
That, and, well...he's a better person that many give him credit for, just not very bright.
His brother on the other hand is worse -- drive Florida into the ground, push No Child Left Behind and ruin education for another couple decades(why not?) -- remove felons(read: Anyone who has the same name as a felon...or same middle name. Or same birthday...) from the voter rolls, 90% of whom just happened to be otherwise registered democrats, pushing Florida red that time...
Now he's going to paint, he's going to try to be like Carter, and he's going to keep his mouth shut for as much as he can about everything else.