This is made quite apparent when you consider their feeding habits. The fact that they eat worms from the sea floor will be delightfully wholesome once they sniff out the scent of a dead, dying or injured sea creature, for it is at this point that things turn from the merely ugly, to the macabre. It uses horizontally moving structures at its mouth and teeth made of keratin (same stuff as hair finger nails and antlers) to get into the body, its invertebrate body tying itself into knots to gain traction. Once a hole is created it can swim in and eat from the inside out, whether its meal is currently living or long dead. As I say, the hagfish simply doesn't care what you think.
By the way, the video above is taken from The Blue Planet, a major miniseries all about life beneath the waves. It's narrated by Sir David Attenborough, which is an immediate Seal of Excellence, and it really is quite wonderful. The full series is available on DVD,
There's also a book, children's book and even the soundtrack is available! It really is a great series.