Earlier quoted context omitted.
The halting problem makes it impossible to consistently answer the question "Will this instruction be executed ?" This is why you can't know if the code is malicious : you can't (always) infer meaningful properties about the code (Will it write something on the disk ?...).
> The halting problem makes it impossible to consistently answer the question "Will this instruction be executed ?" In fact, Rice's theorem says that it's impossible more generally to answer any question about arbitrary code—so no work-arounds like "OK, I can't tell if a particular instruction is executed, but I can just test whether this program is 'safe'" (say, performs no I/O).
Hence we can work around the theorem, by either turning the questions we care about into trivial questions, or by turning questions of the function into questions about the implementation.
For example, we might ask the question "Will program P fire the missiles?". If the answer is yes for some P and false for others, then we cannot answer it for arbitrary P.
We can work around it by weakening the question to become trivial, e.g. "Is program P capable of firing missiles?". If the language allows missiles to be fired, then the answer is trivially yes for all P; if not then the answer is trivially no.
Alternatively, we can change the question to one of implementation, e.g. "Does program P contain the instruction?". Assuming that our programming language contains such an instruction, this is a non-trivial question. However, since it's a question about the implementation (does that instruction appear) rather than the function being computed (which may or may not fire missiles), Rice's theorem doesn't forbid us from answering it.