Book smarts do not equal street smarts or common sense. At all. One can read about something all you want, but doing it is something different. I agree with you that he very likely acted on impulse and had no plan afterwards - however you say that doesn't match your impression of what he was like - do you know him? Again, I think it's much more likely someone like him, dropped out of college, could be smart but have no idea how to navigate the complexities of a murder. Now if he had a long jail record with a track record of being savvy with LE and investigations, I would agree. But to me he seems just the type to not have thought things through all the way beyond the theoretical.
Of course, if he is the killer. Which seems highly likely.
On a separate note, I am just heartbroken at the outcome. Not surprised. My mind immediately went to neighbor and an opportunistic crime.
P.S. - Also regarding him looking like the sketch or not, often times they don't. Second of all, the sketch is not what led to his arrest, it was other evidence so I wouldn't assume it was NOT him because the age is slightly off.