Second degree murder?

Why weren't they all charged with first degree murder? They had planned it for ages. Second degree murder is killing someone as a malicious act but without planning. Everything was planned...so why did some get second degree and others got first? The fact that the wounds that Marty inflicted were more violent doesn't really mean much. Murder is murder regardless of how it's done. Just because the other four weren't involved in the actual killing, doesn't mean they're not as guilty. They ALL played a part in his death.