Puzzles About Life, Death, and the Afterlife

Evangelical Christians tend to believe that we go to heaven when we die and that life in heaven is far better than life on earth. When someone dies, this line of thought is often expressed by statements such as "She's in a better place" or "She's happier than she ever was" or some such thing.

If it's true that Christians go to a better place when they die, does it follow that all Christians are better off dead? (Are we all candidates for euthanasia?) And does that mean that when a Christian dies, we shouldn't mourn for her loss of her life--that we cannot mourn on her behalf, although of course we can mourn because we'll miss her?

Furthermore, a common explanation of the wrongness of an action is that it harms someone. So punching you in the nose is wrong because it hurts you. And killing is wrong because it does someone one of the most signficant harms there is: as Clint says in Unforgiven, more or less, it takes away everything a man has, everything he's gonna be. Can a Christian think about the wrongness of killing in this way? For it may seem that killing a Christian doesn't harm her. It certainly doesn't permanently end her existence. In fact, it may seem that she's better off for being killed. Given what we tend to think about life and death as Christians, why is killing wrong? And why such a serious wrong?