I personally consider prostitution, so long as it is done by choice, the least bad of all of these, because if the prostitute chose that line of work, nobody suffers.
Cannibalism requires the murder (or at least, killing) of people, slavery requires taking someone's freedom away, chem dealing forces people into terrible addictions, and raiding is abusing another person's labor for your own gain.
Of these, I'd say prostitution is by far the least bad, so that's the one I'd consider a necessary evil.