坏人说的话，Words of the evil, 不能相信，cannot be believed. 要是相信了就死定了 if so, you are a dead duck!

I don't know any common English expression like that. Maybe it means something like: "If you believe what the bad guys are saying then you're a dead man" which means that if you believe them then you will be in serious trouble. Is it a line from a film?