The size of the canines is irrelevant. When was the last time you tried to take down, say, a pig with your teeth? The answer, of course, is never. When was the last time you heard of someone taking down a pig with their teeth? Probably never. Our teeth aren't weapons...usually. With the ability to take down an animal and then pull it apart, the need for large teeth is near-nonexistent. Can you imagine having your face stuffed full of the kind of teeth you see on animals similar to normal human weight? We'd need a completely different jaw structure.
Anyway, Humans are Omnivores. This is readily apparent. You can't eat meat if you can't digest it, and you can't eat plants if you can't digest them. Well, you can, but you won't get much in the way of benefits and you probably won't enjoy it. I can't really see how, assuming humanity at some point ran out of gatherable food, they were able to sustain themselves on meat they couldn't digest until they were able to evolve, and a Pre-Human who mutated meat-eating teeth wouldn't know what to do with them, unless the mutation also rewrote his instincts.