The common law holds that a defendant can be found guilty of a criminal offence only if the prosecution can prove beyond reasonable doubt that he has committed the crime.
As a conviction can be secured only on the basis of strong and irrefutable evidence, the burden on law enforcement officers trying to bring a suspect to justice is onerous. And quite rightly so, because the consequence of a wrongful conviction is irreparable.
Stories of policemen posing as shadowy figures to win the trust of crooks in a bid to gather evidence of criminal transactions are often the subject of dramatic screenplays. In real life, a court often hears that a defendant would not have engaged in a criminal conduct for which he is tried had he not been baited to do so.
As the saying goes, you can take a horse to water but you cannot force it to drink. A person who has an absolutely clean background will never accede to obtaining drugs, even if he is lured by a myriad ways to do so. Or so the prosecution argues.
On the other hand, just as a person's blemished past should not be used as evidence against him, a defendant who knows the drug trade because of previous dealings can argue that he would not have done it one more time had he not been prompted. There were even cases of apparently innocent people breaking the law out of love for their best friends, who turned out to be undercover agents.
In the United States, sting operations have been banned. In Hong Kong, entrapment can only be a mitigating factor, but not a defence. Whether Hong Kong should follow the US's way is a matter that deserves to be studied. Meanwhile, law enforcement agencies are advised to consider both the legal and ethical dimensions of their baiting plans.