As an adverb, 死活 emphasizes stubbornness: 死活不同意 (refuses no matter what). As a noun, it means someone's fate/life: 不管死活 (don't care about someone's wellbeing).
grammar
When used as an adverb, 死活 usually pairs with 不 (negative) or 要 (insistent): 死活不说 (won't say no matter what), 死活要去 (insists on going).
