Biomedical engineer here.

If you don't pierce the rib cage, it's a flesh wound to an area not even particularly well-supplied with blood and I'd go so far as to say it's unlikely to be lethal at all, short of infection. I don't think that you could pierce the rib cage with a slash although I don't know for sure; for reference, bones tend to be about half as strong as iron.

If you do pierce the rib cage and hit the lungs, asphyxiation is likely, but slow, not instant. And today I learned on /r/todayilearned that even heart stab wounds aren't necessarily fatal (of course, that's "90% of heart stab wound victims that make it to the hospital survive" - many would die before arrival.)

Short of decapitation, there's not much potential for an instant kill. Humans are remarkably resilient.