How should I approach making Killer Builds?

A good starting rule of thumb, I've found, is never exceed more than two slowdown perks unless all of them flow into one another and you're okay with running a gimmick build lacking in other tools.

It's a trap to think you should run all four slowdown perks; the more you have the less likely you are to consistently activate them, and any slowdown perks you do run are supposed to complement basekit slowdown, not replace it.

From that point, generally what I do is look at the killer's power and figure out both what it already has, and what kind of gameplan goes along with it.

For example, I've been playing Sadako a lot recently, so let's look at her. Sadako has mobility and stealth, but she doesn't have much else; no chase, no info (for the most part anyway), no other utility like an instadown, very little slowdown (but to be fair it does exist).

So on a killer like her, while slowdown is always good and you'd bring at least one perk for it, what she really needs more is chase and info; the former helps her with something she doesn't have, and the latter boosts what she does have by directing you where to best teleport and not waste time.

Evaluating what a killer is lacking and what would support their basic gameplan is a good first step, I think, and you can come to some pretty fun conclusions on that path. For example, I like running Furtive Chase and Nemesis plus something like BBQ on Bubba, because sneaking up on survivors who are suddenly faced with a chainsaw from point blank range is very entertaining and honestly fairly strong— and I came to this conclusion because I realised after one of the old Halloween events that sneaking up before revving is pretty threatening when you can pull it off, the utility of an instadown is heightened by not letting survivors make distance before hearing it start up.

Applying this metric to some other areas can answer some of your other questions, too: Who can best use Hexes? Answering that question, I'd look at what Hexes represent - a timesink survivors have to spend all at once to break the bone - and then start looking at killers with information to know when they start, some form of ability to more easily stop the breaking process, or some other innate slowdown to stack on top of the timesink.

So, Houndmaster's good: she can target totems directly for Search and even if she couldn't, the AOE comfortably covers totem spots, and if she does detect someone she gets a speed boost towards them which helps her more easily stop them to a small degree. Plus, she has an addon that helps protect totems a bit by causing regular screams.
Doctor works here too, Static Blast both tells you when someone's on a totem and resets their progress, plus Madness 3 stops them from interacting with the totem at all. He doesn't have any mobility, but he's still got something that works alongside how totems work.
Pig embodies the third option, the RBTs give survivors enough to do that hunting totems can be a pretty big deal on top of it. No detection and no mobility, but she still has the synergy.

TL;DR, it helps to break killer powers down into what utility they provide, and pick perks that either support what they have or cover what they lack. Also, don't fall into the trap of bringing more than two slowdown perks.