I didn't say there wasn't more stuff going on. But when you get down to it, it's a slasher flick. Hell, it's widely accepted as the slasher flick by a lot of people, fan and industry.
It's like TEXAS CHAINSAW. There's a lot of stuff beneath the surface of that movie about the American family, Vietnam and the advance of technology in the work place, but at the end of the day, it's a slasher movie designed to terrify you. And at the base level, SEVEN SAMURAI is a movie about samurai defending peasants from bandits.
The only reason I can think not to call PSYCHO a slasher is of the fruity 'slasher is a dirty name, and it deserves something higher class than something like that,' hence the more acceptable term of thriller, or horror at the most by some people, with all the connotations horror subgenres like slasher have with folks.
But a big cat with stripes is still a tiger. Even if he went to Harvard, he may be the most intelligent tiger in the world, but he's still a tiger.