Alternatively he targeted an event at this dance studio because it's an enclosed location typically with only one way in or out. If you want to maximise the amount of violence you can commit on a group minimising their ability to escape you and for help to arrive is a big part of it.
At a park people can run. At a single room event unless people go out of the window they have to bypass this stabby shit to get out.
You'll have to forgive me, my knowledge of mentals is limited and my closest interaction with them is the town crackhead who shouts a pigeons and the people in Tesco who talk to milk.
That said, if it's that thought out, with reasoning, fore-thought, pre-planning, after-planning and with no remorse; he didn't 'snap out' of the episode, then he isn't mental, he's just a monster.
Even him saying "The little girls were zombie clones sent by jewish reptilian aliens to steal the thoughts of humans and Odin commanded me to release their souls back to the worlds' tree so they could find peace in valhalla" then, as mental as
that sounds it's at least understandably mental to have a logical path of why he did what he did.
But "bongo stabby stab pale girls because bongo sad" is a deflection. It's worth noting that terrorists et al are counselled during their indoctrination to say they're mental, as it gives them a reduced sentence and better digs post-trial. A cushy mental ward in a hospital with nice grounds and a soft bed. is much preferable to getting shanked by John the Knife while his cellmate fucks the local screw.
The same reason all acts are "alleged" until a conviction is passed. Can't prejudice any potential jury by declaring him guilty and can't risk the potential libel suit afterwards if he's found not guilty by the court.
Unless your name is Lucy Letby. The MSM predictive programming was strong with that one, but it's OK because she definitely murdered babies and if you question it you're a monster with a tin-foil hat and no brain. Now clap like seals for the nhs, it is pure and would never allow multiple tragedies, nope, chud, it was her, the MSM said so.