The spell permanently animates the creature (its duration is instantaneous), so it definitely stays undead.
After 24 hours you no longer have control over it. That means that the DM now has control of it. Which of the 3 the creature chooses is probably dependent on what you've awoken, and how undead work in your particular setting. That said, this is ultimately the domain of DM discretion and which of the 3 options your DM chooses is completely up to him. The RAW here is probably actually the first, but I would definitely reserve the right to choose any of the 3 as they are no longer a thrall and are acting under their own authority.
As a DM, I'd rule that most likely, they are going to cause destruction as Miniman alludes to. But whether or not that directly harms the PCs would very much depend on the situation. If they fail to recast in, say a dungeon where they are the only living creatures nearby, they're probably getting mobbed. If they fail to recast it and they are in a town...well then the town is probably going to have a zombie problem they get to deal with.
I see two broad options, one of which is a bit of a frame-change.
First, the frame change: most undead - especially low-level undead like zombies and skeletons - were mindless in previous editions of D&D and aren't terribly bright in 5E. It shouldn't be too hard for some passably persuasive denizens to convince the undead hordes to follow their commands without magic. Further, zombies and skeletons are frequently portrayed as basically milling about doing nothing in particular if there's nothing better to do; it shouldn't be too hard for the denizens to have corralled the undead into holding areas for later use (it's not like they need food or exercise, after all).
Option 1b, I guess, would be that there's no particular reason for the undead creatures to stop doing whatever it was that they were doing when they're no longer controlled, especially if what they were doing is a mindless, repetitive task ("keep turning this wheel" or "walk along this round pathway (ie., patrol)").
If that's not quite sufficient, the denizens could be keeping a handful of key undead creatures under their control, and trusting that the rest of them will just kinda follow along.
PCs could, in principle, do the same thing: control or convince (or just lure) the undead to where you need them to be, then let them hang out there 'til they're needed. If necessary, keep a small number under control for specific purposes.
The second option would be to adjust (slightly) the Hallow spell, and "Un-Hallow" the area (which may indicate that a more powerful creature is behind this, and the adventure is prelude). As written, Hallow appears designed for Good PCs to use; I don't see why its mirror shouldn't exist, which would allow for necromancers to exert some control over more undead than normal (possibly only just "some" control, though: perhaps it's just enough that the undead see the other denizens as "friendly" as opposed to "lunch").
Again, in principle a PC could do this, if they could find a deity willing to allow it.
Best Answer
Spells do exactly what they say they do. The animate dead spell is a third level spell that animates a single medium or small humanoid corpse/pile of bones into a single undead creature. At higher spell levels, it creates two additional undead per level, but each undead must come from a separate corpse or pile of bones.
What you're proposing is some way to animate a large or larger, possibly non-humanoid creature (for example a giant), from a single corpse/pile of bones by effectively casting animate dead as if you were casting it on multiple corpses/piles of bones. That's not what the spell does and therefore you can't do it this way.
As answered in your previous question, there is still no way to do this officially. You're entirely in the realm of homebrew. One possible solution would be to create a separate spell of higher level that can animate larger corpses/piles of bones, in a way similar to how the dominate group of spells allow control of increasingly more powerful groups of creatures.