Have you ever seen a young, very fit person absolutely off their chops on coke, speed, meth etc etc There is rarely any need to crash and sleep, some people keep pinging around like a multiball pinball for days. Most take some kind of downer to medically induce sleep as they're so amped up its almost impossible. I've seen it, it ain't pretty. Ask any Nurse, medic, Dr, Ambo, Paramedic about what fit, strong, healthy people are like when they've had too much nasty gear and that might clarify some assumptions. He may not have even started 'coming down', he might've upped the ante and got more. Plus there may not have been any real official need for someone to collect their car after or during a bender but they may have been unable to sit still. Stimulants can compel agitated relentless movement. They may have parked somewhere that their car would've copped a lot of bird *advertiser censored*, or a parking fine, or annoyance from someone about where his car was parked-people can be picky about non resident cars taking up space in their street.
I have no verifiable knowledge about the accused's personality or any hypothetical latent tendencies. However I have seen over the years people with no criminal history or antisocial issues suddenly become the perpetrator of a hit and run. Many are intoxicated, some aren't. It doesn't seem that wild a leap to me. Its heinous and unfathomable but not uncommon. Possibly leaving a crime with the injured or deceased individual isn't common. Recently there was the Mother/Son (iirc) near Darwin hit and kill a woman and attempt to conceal her body.
To panic and behave irrationally and inhumanely is not a new trend. And, I doubt what someone may or may not have had planned for a sunny, hot summer's Sunday would've factored much if that morning started the way described.