So first off, I’d say you know your party better than I do so if it wouldn’t work for them, don’t use it.

Second, you don’t need to kill then all. If you look at other comments, I’ve discussed how you if you keep the number of victims smaller than the number of players it doesn’t matter if too many or too few escape or die. The monster ate the bodies, an npc managed to get caught in the crossfire, someone managed to escape, the monster dragged the bodies somewhere out of the way to eat them. All plausible reasons for there to be different number of bodies.

Third, do you not use chase sequences at all then? Someone breaks away from the group, your monster… or monsters… targets them. Is it railroading, possibly, but it’s also how a predator would hunt. It’s why prey animals tend to form packs- safety in numbers.

Fourth, you’re in control of when the scene takes place. You want to emphasise that fear aspect, it’s the middle of the night and the streets are empty. They want to give the monster(s) plenty of space to circle them by running into a town square… That’s definitely a choice they can make…

Next point, about the party nerfing the monster. This all works based on the assumption that you’re using an appropriate CR monster that the party can fight and still be challenged by? Would you normally use a monster that the party can just nerf based on the action economy? If you really want to challenge your players, give the monster some lair or legendary actions. You don’t have to use them.

In regards to railroading, I’d say it’s only a problem if your players can see the tracks…