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.
It ateHercules alive. 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 it's 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, Herc's thread glows and becomes invulnerable with his ascension to demi-godhood at the absolute last second.
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.
Hades: I can't believe this guy! I throw everything I've got at him, and- [Is cut off by the sound of rubber sandals squeaking, as he suddenly glances down at Pain] What. Are. Those?
Pain: [Cut to Pain, who is shown wearing a pair of plastic Air-Herc Sandals] I dunno, I thought they looked kinda...dashing.
Hades: I've got 24 hours to get rid of this...[Flares up] bozo, or the entire scheme I've been setting up for eighteen years...goes up in smoke. [Begins to burn out of control] And you...are wearing...HIS...MERCHANDIIIIIIISE?!
[There is a slurp, and Hades is interrupted mid-rage as he begins to descend on Pain. They glance over at Panic, drinking Herculade] Panic: [Nervously] Heh...thirsty? Hades: [Bursts into flames] GAAAAAAAAAAAAAAAAAAAAAAARRRRRRRGGGGHHHH!