It seems “slain” and “slayed” are both valid as the past principle of slay, though I did have the same thought before I looked it up.
From the linked article:
The alternative past tense and past participle form “slayed” is most strongly associated with the slang sense, “to delight or overwhelm”:
In recent use, “slayed” is also often found associated with the other senses as well. However, this is widely considered nonstandard
It seems “slain” and “slayed” are both valid as the past principle of slay, though I did have the same thought before I looked it up.
From the linked article: