If you were in a persistent vegetative state with no brain activity, you wouldn't have that perogative though. Your family could go months or years with you in that state with no hope of recovery. Some people might see this as a living death, and worse than being able to grieve a sudden death and try to move on.
If you're kept in that state, you're probably kept alive with machines at the families request. When the family decide to switch them off, usually when medical professionals say there is simply no treatment there, then there is no hope, the assailant then is up for murder provided the police have all the evidence and the case in order.
Surely, the impact on the family varies. Before there is an announcement of death, there is always hope, no matter what. We've seen this in cases in recent years, particularly with parents of children who fight in the courts for the right to keep their children on machines when they are being forced to switch them off in the hope of some sort of medical advancements and in fact, some medical professionals do come forward to try, again we've seen this. Nothing prepares a family for death, even when faced with the reality. However, The judge will surely take the impact of victims and families into consideration as with all cases and will adjust the sentence accordingly but it is still a murder, the unlawful taking of a life, no better or worse...
Just finally, A quick murder is probably every bit as every bit as brutal as a prolonged one which the brutal nature often haunts families for life. I think you'd be hard pressed to find a family of any murder victim that says "at least it was a quick one".