Yeah... i just tested this now. You do need the lucky trait. I got arrested instantly (and i made sure the victim was in a secluded place without witnesses). And, holy crap, committing this murder took me like 20 seconds. So its not just a matter of finding a secluded place, but also a matter of time before someone comes in to ask whether youve had sex, haha!
Witnesses don't actually matter in the vanilla game. It's all down to the Lucky trait.
Case 1: Killer is Lucky, victim is not -> Killer gets away, victim dies
Case 2: Killer is not Lucky, victim is Lucky -> Killer is arrested, victim survives
Case 3: Killer is Lucky, victim is Lucky -> Killer is arrested, victim dies
Case 4: Killer is not Lucky, victim is not Lucky -> Killer is arrested, victim dies
It becomes more complicated when you start using modules that apply trait modifiers. It then becomes what is the total value of the killer's Lucky modifiers vs the total value of the victim's Lucky modifiers. Modules like Thorough which perform witness checks are using the presence of witnesses or not to possibly apply a Lucky modifier to the killer.
Edit:
My understanding of how modifiers work is as follows:
Case 1: Killer Lucky modifier = 0, victim Lucky modifier = 0 -> No modifiers applied, so the vanilla Lucky rules above take effect instead
Case 2: Killer Lucky modifier = 1, victim Lucky modifier = 0 -> Equivalent to the killer having the Lucky trait. Killer gets away, victim dies
Case 3: Killer Lucky modifier = 0, victim Lucky modifier = 1 -> Equivalent to the victim having the Lucky trait. Killer is arrested, victim survives
Case 4: Killer Lucky modifier = 1, victim Lucky modifier = 1 -> Equivalent to both of them having the Lucky trait. Killer is arrested, victim dies
Case 5: Killer Lucky modifier = 2, victim Lucky modifier = 1 -> Killer gets away, victim dies
Case 6: Killer Lucky modifier = 1, victim Lucky modifier = 2 -> Killer is arrested, victim survives
If the total modifier value is in the negative, even if the card has the Lucky trait, it's essentially like disabling the trait. Regardless of the value, if the modifiers for both cards are negative then it's equivalent to neither of them having Lucky.