Injection v Adoption

If the UK courts can take a person's child, why then can they not take a life, for surely the former is worse? I know you can't appeal a death sentence once you're dead, but you could still appeal on death row if you had grounds. You can't appeal an adoption order once the child is living with a new family though - all ties are severed. I suppose that doesn't rule out a relationship with birth parents a long way down the line though, when child grows up, but child might not want that by then, or that could even be perceived as worse by any or all parties. As for a life sentence, well, I suppose new evidence might always come to light while incarcerated if you haven't got grounds to appeal right away, but I suppose that should be left up to the man being sentenced - whether or not he wants to sit it out on the off chance, but if he knows it's not going to happen because he's guilty, perhaps he may opt for death!