The term "assault weapon" is essentially meaningless in the US, muddied by the political climate. I would argue, myself, that a semi-automatic rifle could indeed qualify as an assault weapon if it were designed for close-quarters combat; as the AR-15 is a close relative to the military M-16, with the primary difference being a modification (apparently easily undone) to prevent fully-automatic fire, I'd feel comfortable labeling it as an assault weapon. Heck, I'd say the Ruger Mini-14 is effectively an assault weapon even though it was semi-auto and didn't look like one of Rambo's fashion accessories.

I would say one key defining element is the use of large-capacity magazines; there's zero hunting utlility in a 30-round banana-mag, it's there for spray-and-pray shooting whether the weapon's fully auto or not. I would argue that a large ammunition capacity would indeed indicate that a weapon was "made to kill a lot of people quickly."

Canada prohibits magazines of greater capacity than 5, IIRC, except for those designed for .22LR ammunition; those get a 10-round capacity ceiling. As a sports-shooter I never felt the lack. (Heck, most of my shooting was done with single-shot accurized sport rifles firing .22LR; then again, it was shooting for the Rifle team so it was more Olympic style stuff than casual plinking.)

-- Steve certainly agrees with you that there is a lot of sloppy (and magical) thinking about firearms going around in the press and social media.