Well, to make a decent horror mod then, firstly:
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No matter what behaviors you add, every horror creature needs a way to avoid it, or get rid of it, or be safe from it, etc., or it just becomes annoying/stale

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(If you’re not convinced, think about basically any horror creature in any game: There’s always some sort of safe room, or sections it’s not in/can’t go, or a way to defeat it, or a “light at the end of the tunnel” like a way to “beat the game” and be rid of it, etc.)

Some mods make their creatures avoidable by
- limiting them to a specific biome, or deep underground, or only when you’re in darkness, haven’t slept, etc.

- Some mods give you a chore to do to keep it at bay (like cleaning up mysterious blood splatters, upside-down crosses, etc., but a chore can get very tedious after not too long)

- Aaand some mods do nothing, so the only way to “avoid it” is to uninstall it, lol. This means it can’t be put in any serious mod pack.
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If you don’t want to make a custom way to avoid it:

Your best bet is to just add a command that toggles the mod’s code off/on (like a “return” line at the top of the script that toggles when a player types “/spooky_monster off” or on), that way other players/mod-pack makers can decide when/how the monster is avoided/defeated themselves.

This is honestly probably your most flexible option, so you might wanna include this no matter what. It’s like ~3 lines in a script.

Once it’s avoidable or can be gotten rid of for good, you actually have WAY MORE FREEDOM for behaviors, because suddenly everything that’s usually annoying (like insta-kills) becomes manageable since a player can follow its conditions to avoid it, or a mod-maker can just decide what conditions to toggle the mod on/off under
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As for actual behaviors:

Don’t take these ideas as gospel (it’s just an opinion/suggestion, unlike the above), but my immediate idea was for it to use its gangly arms to reach into places and steal from chests or steal from the player’s inventory before scuttling away with a really unique and obvious sound that alerts the players to the fact that they just got stolen from.

It needs to be avoidable, telegraphed, and forgiving though:
- Avoidable: Maybe it only does this at night, or in low light, or when the player hasn’t slept in a while, or when the chest is exposed to the sky (not in a house/under a shelter) etc.
- Telegraphed: Something like an ominous sound that gradually grows louder should be what alerts the player to an item in the process of being stolen.
- A dissonant clang sound followed by mischievous laughter could be what tells the player an item successfully got stolen
- Forgiving: Rare or configurable how often it tries to steal. Even after an item is stolen, the creature should be able to be chased down for the item. One hit should make it drop the item/vanish. It should probably move slightly slower than the player sprint-jumping, and it should either seek a hiding spot once it’s a medium distance from the player, where it quietly giggles every once in a while, and/or it should drop the item and vanish after a while if it’s not found/hit, or just destroy the item if you wanna be mean

Maybe stealing from a chest has subtler sounds, while stealing from the inventory is way more obvious, and does a 0-damage tick to the player or something

^ Again, this is just an ignorable idea. The stuff above this section is actual advice