If you want to be a killer, first you need to choose your fiend deck. A fiend deck is the 3 killers you bring into the match. Although that may seem like a lot, you only play one fiend at a time, so just calm down.

Strangler: The strangler is your average "good but fragile" killer. He doesn't have a ton of health and his ability may not seem that good, but to better players, he's one of the better choices. He uses a chain to grab classmates from far distances and chokes them out, lowering their health bit by bit. He uses vents as insta-kill locations and they have a pretty good reach. The only downside to strangler is that his main attack doesn't do that much damage. He can hold himself in a 1v1 but dies quickly when surrounded.

Slasher: The slasher is a pretty basic killer. He has no special ability but deals the most damage. People don't really fall for his insta-kill spots, and he is really bad with chases. He's most effective with guarding objectives and getting a classmate when their alone.

Giant: The giant is just a tank. He has a ton of health and does a decent amount of damage. His main ability lets him grab classmates to yeet them across the room, dealing damage. He can aslo charge at classmates, grabbing them if they are caught. His insta-kill locations are really stealthy, so if the classmates don't have a scout. you should be good. His charge is pretty bad though, and you cant turn that well while doing it either.

Spider: The spider is also pretty good. It can shoot venom at classmates, slowing them down while also hurting them upon impact. It's insta-kill spots are holes in the ground, allowing for some pretty nasty kills. It also explodes upon death, blasting venom everywhere, hurting classmates around you. It's pretty good in chases but doesn't deal a whole lot of damage when attacking. The spider is good for pushing classmates off objectives and forcing them stop and heal.

When you start the match you should go into what I call stealth mode by pressing Ctrl. It makes you invisible and allows you to move very quickly with left-shift. While in this mode you can place bear traps, plant gas into areas that survivors search for scrap, and activate trap doors and insta-kill areas.
The only downside to this mode however is the fact that you need to get out of the survivors line of site to spawn in again. You cant hurt anybody while using stealth mode unless it's via trap-door.

I recommend that you plant as much gas, and bear traps as you can. Bear traps mainly just slow survivors down but they sometimes step in them if they are hidden well enough [top of stairs,dark hallway,behind doors].

As a killer you are most effective when you single classmates out. Do not engage into a group of classmates like you have a bomb strapped to your chest. You will die quicker than you can deal damage. Even the giant does horribly when put into a group of survivors.

When a survivor dies they are placed in a room on a random part of the map. They cannot spawn in again unless they are saved by another player. Usually only one classmate goes to save someone. Take this time to guard the rescue room and get another kill. This doesn't mean "forget about the other classmates while they do the objective". Your job is to apply pressure. If the all of the classmates go to rescue, take that time to plan and defend the objective.

In order to win you must either kill all the survivors or prevent them from completing their objectives. In order to kill them all, you must eliminate all of them until there is nobody left to rescue. Preventing them from completing their objectives may sound easy but that doesn't mean "just try to kill them all as fast as you can". Survivors have a time limit to complete these objectives. Just constantly kill the classmate doing the objective and you should be fine. This will cause their plan to crumble until they end up just trying to kill you.

I hope this somehow helped you become a decent killer. If it didn't then just look up a guide online or something...