Yes they are lethal. If you hit somebody in the head or torso repeatedly. But they will not sever arteries. They can disable limbs though and that's what makes them effective for self defense. Too bad they're illegal mostly though.
Never carry a knife on you with the intent of self defense, unless you are completely willing to take the life of that individual. And if you are completely willing to lose your own life as well.
Why would you laugh if a criminal pulled a knife on you? I'm talking about law abiding citizens trying to protect themselves with knives(and it being a stupid idea).
An actual serious criminal isn't likely to brandish a knife if they intend to harm you. You won't see it until you're already disabled or dead.
Lies about knife fighting
Do you know what a prison yard rush is? Yeah, that's how you'll most typically die to a knife from a criminal.