It's not about the rape, it's about the murder. I can understand what you're saying about gang raping them, but I'm more referring to the murder aspect of it. Besides, she died almost immediately after being raped, so it's not like she had to live with the pain of being gang raped. I think that in this case, the crime of murder basically overrides the crime of rape, so we should charge them for murder instead.
And yes, if 1000 people all murdered 1 person together, I don't think that all 1000 people should get life imprisonment. After all, they could have murdered another 999 people and still got the same punishment. We don't want to put people in a situation where they say to themselves "I've committed one crime and I'm about to get the death penalty, so I might as well just go on a murdering rampage, since the punishment is going to be the same for me regardless". That is why the punishments need to be more appropriate. I mean, as long as they are spending some time in jail + given a criminal record which will basically ruin their future, then that's enough.