flammable 可燃 and inflammable 易燃 both mean “able to be set on fire.” Why in the world is this? Do the language gods have a cruel streak倾向?

There is a fairly clear reason for why both these words carry the same meaning: the prefix in- does not always function as a negative prefix. Sometimes (and this is one of those times) it serves as an intensifier.

Though in the recent past flammable is used more often than inflammable.

Flammable = able to catch fire
Inflammable = high likelyhood of catching fire

其他带有IN前缀的类似例子还有

Invaluable 无价的= exceptionally valuable 非常有价值