Throughout the history of the English language, verbs have ended to become regularized- that is, to take -ed as the past tense and past participle form. The verbs that are used more commonly, tend to remain irregular- like go, went, gone and eat, ate, eaten. The verbs used less often tend to regularize. So "dive" can now have the past tense "dived" or "dove". "To slay" is not a commonly used verb, so it has regularized to "slayed".