large poisonous snake
虺 refers to a large, venomous snake. The meaning is specific to a dangerous serpent.
虺 combines 兀 (a pictograph of a person with a bent leg, here possibly suggesting a raised head or striking posture) with 虫 (insect/reptile), together indicating a snake. The exact historical development is uncertain.
The outer 兀 looks like a raised, threatening shape, and inside is 虫, a creeping creature. Picture a large snake rearing up, ready to strike with its venom.
For キ, imagine a key-shaped fang on the large poisonous snake: key -> キ, and the snake's bite is the key to its danger.