Perhaps this word has another meaning in the berserker-Viking sense, but it's certainly not "love of death or dying". "Mord" is the word for murder, "Lust" meaning pleasure/joy/appetite, often with the implication of carnal pleasure. So: love of killing, or even love of killing to meet a perverse sexual agenda.
The modern usage of Lustmord, in my admittedly limited experience, agrees with my Cassell's German-English dictionary: "murder and rape, sex murder."
The specific class of Viking that consumed Fly Agaric was the berserker class. Legend has it that they were prone to invincible bouts of madness and shapeshifting, that their powers were rooted in anything from the power of the drugs to epilepsy to "genetic defect"; though their potential on a battlefield was immeasurable, they frequently slayed their own people in their fits of berserkergang, and were purportedly prone to, specifically, and in the unmistakably sexual sense, rape.