Situation: Generator 80% complete. What do you do?

Are we talking at the beginning of the match or at the end of the match?

You can't stop generator progress as a killer unless you're a really good Billy or Nurse. If you stick around one generator, survivors will just move to another one. Guarding a single generator is essentially the same as face camping but way worse since all survivors are up; all your gens will get completed because you have no control over the map. Your best bet is to pick 3 generators really close to each other and make sure those don't ever get worked on. One of the worst situations you can be in as a killer is having 3 generators spread out across the map.

Really you just need to feel it out. If you went to the rest of the generators and all were worked on but the last one which is almost complete, then obviously the survivors are hiding in that area. They aren't going to want to leave, so your best bet is to kick the gen, do a quick look around bushes/trees whatever to see if you can find a survivor. This is where Whispers come in handy. If you can't find them, move on.

@Wahara said:
As an aspiring killer, I have only made it to rank 7 but of course am interested in changing that. There's one particular situation that I find difficulty in deciding what course of action is the wisest to prioritize. There are a lot of games, especially on the larger maps, where the survivors all spawn next to one generator. Unlucky for me, I've check every other generator before I find the one they are working on and it's nearly complete.

My instinct is to stick around for awhile after I've scared everyone off, or fake going after someone else and come right back, because there is almost always one or two people hanging around to pop that generator as soon as you take off. While this seems like the proper course of action, I find it only pays off with about a 50/50 chance. Sometimes, I can down someone by faking the chase and returning to defend the nearly completer generator. Other times, two other generators become complete by the time I've chased people swarming off the one that was nearly complete. What do you do? Is there anything you can do, really, save for leave it up to chance?

That early in the game I would hunt someone. If you're having difficulty with people looping you then lose that target and look for something else.

It really all depends on your situation(perks,killer,addons)
If you have more of a chase style build then try hunting someone, but if it's mid game and you're running some regression perks then I would bump the generator and look for my target. Try to at least injure them. Be very aware of your surroundings. You usually run faster than the survivors. So, say if you're by a very end of the map generator where you're at dead ends and there's only one way to actually leave that little area. Observe where your prey runs. They can only run so fast, and usually have to cover some ground before being "safe" from you.
If you can bump a gen and possibly injure them around the same time of doing that, it might be worth your while to pursue them.
I hope that doesn't sound confusing. You really just have to learn what to pursue based on your situation. There's a lot of factors going into it.

@Wahara said:
As an aspiring killer, I have only made it to rank 7 but of course am interested in changing that. There's one particular situation that I find difficulty in deciding what course of action is the wisest to prioritize. There are a lot of games, especially on the larger maps, where the survivors all spawn next to one generator. Unlucky for me, I've check every other generator before I find the one they are working on and it's nearly complete.

My instinct is to stick around for awhile after I've scared everyone off, or fake going after someone else and come right back, because there is almost always one or two people hanging around to pop that generator as soon as you take off. While this seems like the proper course of action, I find it only pays off with about a 50/50 chance. Sometimes, I can down someone by faking the chase and returning to defend the nearly completer generator. Other times, two other generators become complete by the time I've chased people swarming off the one that was nearly complete. What do you do? Is there anything you can do, really, save for leave it up to chance?

If this is at the start of the match then continue with your tactic to quickly find a survivor but then focus on them. The only real "pressure" a killer gets is by getting a hook and doing your best to snowball from it. If you manage to hook someone and start a chase on another survivor possibly getting them downed around the same time that other person is saved then you are keeping pressure on survivors and pulling attention away from generators. The only time you should really stick to a gen that close to being done is if its endgame and you truly feel the survivor is hiding nearby (even then don't spend too long) or you have a specific gen build.

As many others have said, its extremely situational and there's no best overall course of action, but it does help to learn of new options that can help you read a situation better, and make a more informed decision to chase or stay.
Typically I'd kick the gen, but then go chase someone and consider that gen a loss, since continuing to hover around it at best forces all of the survivors to move to a different gen to work on and at worst 3 of the survivors will move on to a different gen and one will hover around and wait until you eventually leave the area and just pick back up on the gen with whatever percentage of the gen is left.
Add in the fact that survivors don't even have to touch the generator for a full second to stop regression, and you've pretty much set yourself up to lose (as a survivor main, I tend to do a lot of "quick touches" on kicked generators mid chase if I know the killer is obsessed with keeping it regressing, since it will take the killer way way longer to kick a gen to start it regressing than it does for me to tap it and stop the regression, making it a viable escape tactic in many cases).

With that said, it always helps to learn new options on how to deal with these situations, so the next time you run into them you can more easily assess the situation and make the best choice with the information you have, and a good way to gather that data even before reaching that first generator is to pay attention to the loading screen, if most or all of the survivors are running tool boxes, you can almost gaurentee they'll stick nearby after you kick it, since pretty much an regression you create by kicking the generator can be undone pretty quickly with a good toolbox.
You can also pay attention to how the survivors flee the area once you approach.
Are they all using SB? well then they probably are heading for a different gen since they have the momentum anyway, so get you kick in on the generator and get back to chasing them in the direction they went.
Are one or more of them sneaking around with urban evasion? odds are someone is hovering in the area, so better to lose one gen but secure one kill rather than lose both.
Of course if you can easily see them all hovering around the area, you can either bait them by leaving the area and coming right back, or once again, chalk that generator up as a loss and just secure what you can, or just keep smacking them as they try to dive for the generator, but unless you know the survivors you are facing aren't very good, its just as likely you'll get played by one survivor while another one pops in to stop the regression.

Hopefully some of that info helps