Laws like this are seriously difficult to deal with.
This is a law that was introduced back in the days of duelling, when it was thought that duels would decrease in popularity, if seconds and doctors in attendance could be charged with murder, even though they were not directly responsible for a death.
Obviously each side has high emotions - the victims' families feel that the change is unjust, and will allow accomplices to be released, and the convicted prisoners' families will feel that denied justice is now available to their loved ones.
As I understand it, the appeal will have to show that the convicted accomplice was badly served by imposition of the law - it will not simply be a (literally!) Get-Out-Of-Jail-Free Card for everyone convicted under its application.
So, when the furore has died down, we will have to see how the law is applied, and it will be to each case on its merits.