Some types of games simply can't be run in a traditional RPG very easily. The old idiom "a period of suspense or building tension is simply time to reload" when it comes to RPG's. Add to it that most gamers are smart enough to know that mechanically it is ridiculously silly to split the party.
For Horror one shots I use the following rule that is simple to implement and makes games much more closely follow the path of horror movies.
The Monster(tm) rolls a number of dice (for any die roll) equal to the number of surviving party members at the start of the encounter and picks the best result. The party rolls a number of dice equal to half (round down to 1 minimum) the number of surviving party members and picks the worst result.
Why is this good?
Much like in a horror movie, people are helpless mooks against The Monster(tm) as it downs its first few victims. It also means that its a bad idea to have everyone pooled together in a phalanx position since its most beneficial if someone is killed all on their own (The Monster(tm) has its dice roll benefit based on the start of the encounter, this means if it kills Bob outside the boathouse and sneaks in to kill Sally, those are two encounters and Sally's odds have just improved).
Your best bets to take down The Monster(tm) are one or two final survivors, hopefully with some of your fellow players having slowed it down previously.
I also allow each dead player to give a single re-roll at a time of their choosing. Either by giving another PC a second chance or forcing The Monster(tm) to re-roll a good roll. This is to alleviate some boredom after they are dead. The other option I've used is to let a random dead PC control the monster's hack an slash mayhem each encounter.
Other things I do depending on system? Either remove luck points (piecemeal) OR if using Hitpoints (a d20 variant) have the monster's damage be multiplied by the number of PC's.
How do you run Horror?
Bonus Episode 69 – situational GMing
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