It is not for judges to create criminal offences, but rather for the legislature to enact such offences. This has been true since Frey v. Fedoruk, 1950 CanLII 24 (SCC),  S.C.R. 517, and is a precept which finds statutory recognition in section 9 of the Criminal Code.
A person may not be convicted of an offence at common law. [FN1]
While the Criminal Code removes criminal liability for common law offences, it maintains the availability of common law defences, except where they are inconsistent with subsequent legislation: see section 8(3), Criminal Code.
While some common law defences (eg. defences necessity, duress, and entrapment) are well-understood, other common law defences remain to be discovered.
In 1993, the Quebec Court of Appeal recognized the very narrow common law defence of suicide pact. This defence is available only when the parties formed a common and irrevocable intention to commit suicide together, simultaneously by the same event and the same instrumentality, and where the risk of death was identical for both.
See R. v. Gagnon (1993), 84 C.C.C. (3d) 143, 24 C.R. (4th) 369 (Que. C.A.).
This is different from a murder-suicide pact in which one person has agreed to first kill the other and then kill himself. Gagnon did not extend the suicide pact defence to the murder-suicide situation.
In R. v. Dobson,2018 ONCA 589, the appellant sought to have Court of Appeal for Ontario go even further than the Quebec court did in Gagnon and recognize a common law defence in situations of a murder-suicide pact. The defence would not be a complete defence; rather a partial defence, reducing murder to manslaughter to reflect the culpability of the survivor of a suicide pact.
The appellant faced two problems from the onset: first, the defence was not argued at trial (rather, only on appeal); and second, the defence (or even the suicide pact defence) has never been recognized as being the law in Ontario.
In addition to these two obvious challenges, the Court in Dobson took the position that the question of whether a person who kills pursuant to a suicide pact should have a partial defence to murder is probably a matter that is much better addressed by Parliament.
R. v. Dobson, 2018 ONCA 589, at para. 44.
While the common law is dynamic and flexible, it is the legislature and not the courts which has the major responsibility for law reform; and for any changes to the law which may have complex ramifications. Where policy questions are so deeply embedded in the legal doctrine the court is being asked to review, the court may be in danger of reforming the law on an inadequate appreciation of public needs and public opinion.
See for instance, Lynch v. Director of Public Prosecutions for Northern Ireland,  1 All E.R. 913 (H.L.), Lord Kilbrandon, in dissent.
This was likely such an instance.
[FN1]: the offence of contempt is an exception to this.