Every killer is rated in terms of 7 categories. These are not equal in terms of power if you go by tier lists, but instead to get a feel for how killers compare to each other in terms of strengths and playstyle. For example, a high stealth rating doesn't translate to a high tier list placement.

The baseline of C rating is the typical M1 killer. Normal terror radius, movement speed, and no power. Think of this as a Trapper without traps. Anything above a C rating is better than normal. Anything below C means something about that killer penalizes them in that attribute. An E rating is completely unplayable in that attribute. An S rating is borderline overpowered at it.

Aptitude Key:

Mobility - How fast you move across the map between objectives. Short speed boosts don't really factor here unless they're spammable.

Chase - 1v1 ability. How quickly you win chases and down survivors.

Slowdown - Your ability to slow down or disrupt survivor objectives. Killers with natural slowdown(such as Pinhead or Pig) have a higher rating. Killers that take time to set up(Trapper) or tier up(Myers) their power will be lower.

Info - Tracking and information. Aura reads, killer instinct, etc.

Defense - Ability to contest/defend generators or unhooks, either by forcing unfavorable trades or blocking off areas.

Stealth - Ability to hide your presence and ambush survivors. Generally factors terror radius, undetectable, and model size in some cases(Chucky).

Snowball - Burst potential to quickly swing a game or win outright. Things like instant downs, ability to injure multiple survivors at once, or ability to instant-kill. Also mildly factors things which allow you to disrupt second chance to secure kills(Pyramid Head cages), since Tunneling a kill often starts a snowball.

Killer Strengths Visualization