Dead with 5 HP left? I did have 5 lifesteal. The combat prediction said 'safe', although I did attack twice in a row so it might not have updated - but the initial combat prediction showed me with more than 1/2 HP remaining after the first strike, so I assumed I would have enough for two hits. (Although I've learned that the HP bar in the combat prediction is not always reliable when you have lifesteal.)
EDIT: More combat prediction weirdness. After attacking the Burn Viper was, indeed, killed, but it used up my death protection to do so. I think maybe this is due to some weirdness with lifesteal?