I think regardless of whether drugs are being over prescribed (which is completely plausible even without references simply because world-wide there's an increasing trend towards powerful pharmaceutical lobby groups and lifestyle drugs) we should look at the reason why they're needed in the first place. Drugs or not, there's a pre-existing condition being exascerbated to the point of someone murdering another person.
I think it's the classic inability of Japanese culture to deal with undesirable things like mental illness, personally. My Japanese isn't great but I was listening to a university radio station the other day and a bit came on about seishinbyou 'mental illness', and again I didn't catch all of it but the speaker was very clear about the first barriers being those created by family and friends who deny that someone close to them is ill and thusly deny that person the support they need to thwart mental illness when it sets in. Having someone to talk to is a huge piece of the puzzle that many of us open Westerners with our large social circles take for granted.
Both times I got over here I ended up breaking up with my gf whom I'd left behind in my home country (I suck at long distance relationships) and not having my friends around me was incredibly tough. I felt isolated and alone when I needed my buddies the most. Luckily I had made some new friends (as us Westerners are apt to do quickly, socially we're like rabbits or cockroaches) and I had a decent support network. Some Japanese people live in such utter isolation and solitude it'd almost fit the very definition of mental illness itself, let alone any actual mental illness issues they may have. Wake up, travel 1hr to work on crowded faceless train, work *** off, late, come home, sleep, day in and day out, weekends catch up on sleep, watch TV, sit quietly in apartment, repeat, wash, rinse, repeat. I'm not saying its everyone but there are some people out there who live like this.