it is hard to imagine this theoretical situation. children who will be murderers... without a chance to change the way they will go, it is too much incredible condition. That supposes that some kinda *extra-magic-exceptional* ability exists: ability to predict the future. but at the same time, there is no *quite an ordinary* ability to change something in the life/upbringing of that children to prevent them from becoming murderers, already knowing their inclination for crime. That is just absurdly IMO. That is similar to the supposition, that a doctor knows that his patient has susceptibility towards diabetes, and due to that fact decides to kill him, instead of performing prophylaxis.
as for the other part of the question, I am sure that some assholes got to be killed for the shit they had done to other people. there are such an assholes in this world, that all that progressive philanthropic charitable & whatever-else-it-can-be-called views do not stand up to any criticism!
sometimes I think that some people are just up in the clouds, talking about cancellation of capital punishment... just read some criminal chronicle of events!
So, coming back to the subj, indisputably, if I ever face that choice - to kill a bastard, or not to kill, I will do it. but only when it is about people who had already done some deadly sins, not about children who "will grow up to be..."