I always thought it was "Kill your mother, kill your cat".
Anyway, I've heard this shit for years. Like in high school I overheard someone talking to one of her friends about how metal bands are all about killing animals, satanic sacrifices, ect..., and it kind of bothered me. So I took it upon myself to explain to her that her logic was flawed, pointed out the artistic merit of certain bands lyrics and the ideologies behind so-called "extreme" lyrics, and pointed out that I, myself am a vegetarian and wouldn't support a scene if it promoted such things as "killing kittens". We became good friends after that.
Maybe "metalheads" just need to explain these things to those that don't understand rather than insult them or ignore them the way that I usually see. Not everyone is a complete moron, they're just misinformed. Of course when you have idiots like the guy from Norway carrying a head around at parties, or this shit I remember a good 10-12 years ago on the news about some guy killing his own dog (cut it's tongue out and it bled to death) in some lame sacrifice because he thought it would impress satan and David Vincent, coming along and ruining things, it sets us back a bit.
On a side note, there was a funny bit where they interviewed David Vincent and asked him if he believed in satan, to which he replied "I am satan." (obvious sarcasm). Haha. The interviewer just sat there like an idiot.