I personally think the law is being misused in this case. Firstly, there's two separate aspects of law being considered here.
The first is felony-murder - or the act by which an accidental or unintentional killing is upgraded to murder.
The second is conspiracy - or the act by which the total consequences of a group action and equally shared amongst all members of the group, regardless of which individual committed which specific act.
The reasoning for each of these circumstances is very clear.
In the felony murder case, the basic principle is as follows:
If, during the commission of a felony, you commit an act with disregard for the safety of others, and are aware that said act could result in death, you are guilty of murder rather than a lesser homicide charge.
The text book example is the guy who is racing away from a bank robbery in a car, and runs through a pedestrian crossing, killing a pedestrian by accident. Normally this would be some lesser driving charge such as dangerous driving causing death or vehicular manslaughter or something. However, in this instance your willful disregard for human life was a direct consequence of your efforts to successfully commit a felony (robbing the bank) thus the crime has been upgraded to murder.
To me, this is entirely legitimate.
The conspiracy theory works in the same way. The basic principle is:
All conspirators to a crime are equally responsible for any of the actions of any member of the conspiracy, during the commission of the crime.
In other words, once you agree to cooperate with a group of people in order to commit a crime, you become responsible for anything any member of that group does. So if Tommy goes ballistic in the bank and starts shooting people, you're all responsible.
The main purpose of each of these crimes is twofold. Firstly, as a deterrent, it attempts to prevent crime escalation by encouraging criminals to be more careful. Fact is people will commit crimes, people will conspire. At least if there's a death sentence over their heads they might be a bit more careful about making sure no one is killed during the crime.
The second is punishment. Committing the crime is bad enough, but knowingly allowing the crime to escalate (by letting a fellow conspirator go mental, or by racing away from the scene like a lunatic) put the public in increased danger, all so you could pull off your illegal act.