Once upon a time, there is a man in his tower, that only he can manage. His machinery keep the world from falling apart and saves millions of lives. Everyday, thanks to him, he cures disease, famine, even some wars were prevented thanks to him and his precious machinery. Millions live everyday secretly thanks to this man, for reasons no one except him can tell. Only him know how

But here is the catch, when his work is done, when his tools are down. During night,
He becomes the other part of Mister Jeckyll...Mister Hyde a monster he can't control 
Like a shadow in the streets. He kills. He steals. He defiles. Not often. Not widely. 
But it often hurts poor innocent people and always... with cruelty. 

One night, a proud policeman catches him guilty after a barbaric act. 
Hands red with blood, from a fresh murder. There is No Doubt this monster is guilty the Justice can be dealt right now.

But here the double edge. 

IF the law acts, IF the man is punished for what he has done, the hidden knowledge and all the work he does vanish. With him down absolutly no one can carry the system he made. It just dies. It crumbles on itself. And with him down, millions will suffer, slow and sure. They'll be down with him. Maybe not in an instant, but surely tomorrow or the week after. 

So what do you do now? If you were the policeman ? 
The gun in your hand, and HIM in front of you, with bloody hands ? 

Do you let a murderer slip away because he is one of the secret gears of the world ? Or,
Do you strike him ? Knowing the punshiment you do right now will punish millions of people all accross the world, also condemning them to a certain death?

How many deaths does it take to justify one spared life? and vice versa ? 

And worse. If you let him go, what does that make you?

Would you prefer become:  
The Good Devil that let a Bad Angel alive ? 
Saving millions from a slow death, Condemning few with brutality? 

Or The Bad Angel that arrest a Good Devil ? 
Condemning millions to a slow death, Saving few from brutality?

Because after all... Hell is paved with Good Intentions.

Note : Then we can imagine a lot of variation :
what if it's the wife of the policeman that the badman killed brutally ?

what if the wife is into the million of people the badman keeps them alive ? 

what if the policeman himself who is into the million of people the badman save ?

what if the policeman is the next to be murdered brutally ?  

Thanks for your time