The jury in the Becky Watts trial has been given directions on what they must be sure of to reach guilty verdicts.
1. Whether Nathan Matthews is guilty of conspiracy to kidnap with Shauna Hoare and vice versa. The crown must prove that:
There was an agreement between the pair that Becky would be kidnapped.
There was an intention that both of the pair would play some part in the kidnap.
...
Whether Shauna Hoare is guilty of murder or manslaughter or none. The crown must prove that:
That Hoare participated in the kidnapping of Becky Watts intending that she should be killed or come to serious bodily harm to overcome her resistance.
In the course of the attack Matthews suffocated or strangled Becky while intending to kill her or cause her really serious bodily harm.
OR if Hoare participated in the kidnap of Becky, and reasonably believed that some harm might be caused to her this is manslaughter.
http://www.itv.com/news/west/2015-11-09/becky-watts-trial-what-the-jury-must-decide/