When a policeman\'s family is kidnapped by a sadistic crime lord he must kill the chief of police, his father in law, to save them.

I would agree with the above comments. It tells us exactly what we need to know in a short sentence, and gives us stakes, drama, a hook, and clear goodies and baddies (excuse the terms). I can see where the others are coming from in terms of ending on a stronger word plus perhaps adding in another twist so it doesn't sound like your normal cop vs crime lord movie.

The log line works just think it needs a little spicing up.

Thanks guys both excellent comments and I'll use the advice in both of them

I like this logline.

Sometimes it helps if you end on a strong word. (I will add in Phil's, honest cop, suggestion)
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"When an honest cop's family is kidnapped by a sadistic crime lord, he's ordered to murder the chief of police; his father in law, or they will be killed."
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This way the logline ends on the word 'Killed' instead of 'them'.

But ultimately this logline is strong, I would suggest only minor changes.