Final answer:
Without additional details or evidence, it is impossible to determine who might have killed Crystal if Carl did not; it could be Joe, Lila, someone else, or it might remain undetermined.
Step-by-step explanation:
If Carl didn't murder Crystal, according to Joe and Lila, we would need additional information to ascertain who might have killed Crystal. Without further evidence or testimony, it is impossible to determine if it was Joe, Lila, someone else, or whether it cannot be determined based solely on the information provided. The determination of a murder suspect requires careful consideration of motives, opportunity, and physical evidence, which have not been presented in this context.