For over a decade, the United States has maintained a 'Kill List' of individuals it deems a threat. With the stroke of a pen, President Obama has repeatedly acted as judge, jury and executioner for thousands of individuals. So controversial, the process for deciding the Kill List has reportedly been dubbed "Terror Tuesdays" by White House aides.

On Monday the U.K. government joined the U.S. in its own form of Terror Tuesday. The prime minister calmly stood before parliament and declared that he and the secretary of defence had ordered the killing of a British citizen. Refusing to release the legal justification, he didn't stop with just this killing. He and various members of his cabinet then declared that the U.K. was considering further targets as far afield as Libya and that they wouldn't hesitate to launch more attacks.

The public was given no evidence, legal or otherwise, to justify the killing. Instead, the prime minister simply asked the public to trust him. The killing was legal and the individuals posed a direct threat to the UK, we're told.

'Trust us' is not how democracies work, however. In Britain at least, the government has a duty to be transparent about its actions, so that the public can rightfully judge the lawfulness and morality of actions taken in their name.

The British public now deserves some answers, starting with why David Cameron thinks his decision was legal. There is no reason to refuse to reveal the legal details of the policy, as Cameron is doing, and every imperative for the government to come clean about its plans for further targeted killings.

The prime minister also needs to start defining the terms he's using. In the War on Terror, definitions are important—and famously malleable. The U.S., for instance, has defined "imminent threats" to be anything but "imminent." In fact, under the U.S. definition of self-defence, the U.S. needs neither "clear evidence" nor for the attack to be in the "immediate future". So, in effect, imminence is anything but. Is this how David Cameron also now defines self-defence?

And what about the intelligence behind such strikes? If indeed, an attack were imminent—and the intelligence so good as to justify a targeted killing—why couldn't the U.K. simply arrest the individuals in the U.K. working with Khan? These were the individuals who posed (or perhaps still do pose) the greatest threat to the U.K.

The prime minister should also consider the difficult questions raised by the U.S.' own Kill List. Reprieve's research has shown that on average, it takes the U.S. three attempts to kill one person on its List – in the process, dozens of others are killed. Was this the first time the UK targeted Khan— and if not, how many others have mistakenly been killed in an attempt to kill him?

Perhaps more importantly, though, the prime minister needs to explain why he has chosen to adopt a model for fighting the War on Terror that even President Obama's own generals have said isn't working – and may be increasing anger and resentment against the West. Given what's at stake, 'trust us' doesn't cut it.