How deadly are black holes?

Answer 1

Close proximity to a black hole would be fatal to a human from a gravitational effect known as spaghettification, which is basically a tidal effect which would stretch you into a long thin string. Friction or crushing effects from other infalling matter could instantly cook you or turn you into vapor; radiation from the accretion disk could also be so powerful as to instantly kill you from radiation effects which might once again sublimate you to vapor.

For larger black holes with less noticeable tidal effect and even without an accretion disk or relativistic polar jets might allow you to survive entry past the event horizon, but there would be no leaving the black hole. For an outside observer it might seem time would dilate significantly and you would seem to slow to a near-stop approaching it, but for you time might seem to pass normally and you may not even be aware of the moment you cross the horizon.

While black holes consume all unfortunate matter in their path, they are also thought to be vital to the formation and stability of galaxies including our own (supermassive black holes) - so in one sense, black holes are essential to the behavior, large-scale structure, and stability of most visible matter in the universe, and given our favorable position in the galaxy one might also say our very survival is interconnected with black holes.