The Titans are so scary, they almost look like they were coming out of the screen.
The Titans. They were heartless killers, and it was implied in the opening song that the remains of their victims WERE LITERALLY EVERYWHERE. Considering they're portrayed as little more than mindless, hate-filled forces of nature that could not be reasoned with at ALL, only locked away, you can imagine that Hades would probably have had trouble keeping them under control if he had won. The only reason he got them to obey him was that he had a target for them, once Zeus was gone, who's to say that Hades could control what he had unleashed?
The Hydra: it's monstrous, no pun intended, and what makes it scarier is that it's like the one monster that gave Hercules trouble - even more than the titans, it was the only thing that came close to killing him.
And it's a good thing Herc happened to have his sword on him at the time it swallowed him.
It also stops to laugh at Hercules, and takes its time trying to eat him again when it has him pinned to the cliff, so its also a kinda sadistic monster.
Meg is seen dying in agony and then shown clearly as dead as Disney can get away with on a G rating.
Hercules' rescue of her soul showing his rapid aging gives both kids and adults nightmares.
As if these two aforementioned examples weren't enough, the Fates attempting to cut both threads of life with scissors is enough to make viewers young and old cringe and hold their breaths in fright of what would happen. The first time, they are successful in cutting Meg's thread; the second, they become unsuccessful at the moment Herc's thread glows up as the scissors start touching it on his rescue of her spirit.
For that matter, Hades getting pulled into the dead pool.
Hades' mood swings.
Hades, when he's angry, goes from controlled blue flames to out of control red flames. Hope you aren't terrified of fire.
His angry face is no picnic either.
Just imagine watching the close-up on the big screen as a kid.
The most angry time is when Pain and Panic are using Hercules Merchandise...
Hades:(to Pain, working himself up) I've got 24 hours to get rid of this.. bozo... while the entire scheme I've been setting up for 18 years goes up in smoke... and you... are wearing... his... MERCHANDISE?!
(There is a slurp, and both Pain and Hades are interrupted mid-rage. They look at Panic drinking Herculade)