If you want an old character to be awesome and enjoyable, you can give the character some personality quirks (eccentricities) that make the character unique, unexpected, and hilarious (see “2 Types of Kick-ass Old Characters That People Love“). So what do you do if you want to make the old person (or any character) creepy and horrifying? Easy. Give them personality quirks.
Yeah, it sounds like a contradiction in terms, but it’s true. Personality quirks can make a character awesome or creepy or annoying (or something else). In fact, the same personality quirks could do any of the 3. For example, saying whatever comes to mind:
“This boot makes a great hat!”
Let’s say that Aunt Mabel (who can’t sew but loves quilting bees, bakes delicious cookies for visitors, and would never hurt a fly except in defense of her houseplants) said this as she pulled a boot out of the closet and stuck it on her head. Assuming that the characters around her shake their heads and chuckle, the audience is liable to be amused, too (unless this is simply not their style of fiction).
On the other hand, if a maniacal serial killer (with blood splattered across her face, a dead body at her feet, and a big toothy smile) said this as she pulled the dripping boot off the corpse’s body and puts it on her head, it’d be a whole other story. The characters around her wouldn’t laugh unless they were hysterical or equally twisted, and the reader is more likely to be shocked, disgusted, or horrified than amused.
And if the main character’s kid sister (who never shuts up, has the energy of a humming bird, and has the bad tendency of following the main character around and ignoring any hints or commands to stop) said it in a store as she grabs a boot off the rack and sticks it on her head giggling (after doing similarly time-consuming and pointless stuff for the last half-hour), the main character’s more likely to explode with irritation and spent patience – and the reader will probably cheer.
These are 3 very distinct reactions to the same statement made by a female character. The quirk of blurting out whatever she thinks is the same, too. But that’s only a minor facet of the characterization. Core values, context, actions, and character interactions are all bigger parts of the character than his/her quirks. Those big details shape the reader’s response far more than the eccentricity alone.
Think of eccentricities as an optional spice in a recipe. Yes, having the spice and putting it in can make the recipe better. But you can make the recipe without it. You can’t, however, make the recipe by adding that spice and leaving out other non-optional (A.K.A. essential) ingredients.
So you can’t rely on a quirk alone, and you need to make sure that the character building backs up the quirk for the effect you want. The first (a character with plenty of eccentricities but no real depth) ends up becoming a caricature instead of a strong character. The second option could turn your lovable, hilarious, and awesome kung fu master into a creepy, villainous, or annoying kung fu master that the main character really shouldn’t listen to. It’s a surprisingly easy transition.
Long story short: eccentricities are great, but character building is everything.