And what are you going to do once you have that bitcoin in your secure offline wallet? Anywhere you spend it can see it came from your address and you just kidnapped X person, so you'll have a hard time spending it in person especially if authorities are watching. You can try to spend it online, but if you spend it with anyone more easily accessible than you and provide any identifying information to convert funds to a local currency or ship yourself something, you're screwed.
Like I said, the best you can do is mixing and similar, but you know what criminals actually primarily use? Cash. Cash is fungible and untraceable. If you're going to do some hostage taking, you may as well do it for cash rather than bitcoin, the only potential benefit of bitcoin would be verification might be easier than cash.
Ransoms are not paid with clean cash. They are paid with marked bills. If you take a ransom and, after taking some precautions, wait a year then meet up with someone in another country it's pretty likely you'll get away with it. If you have someone you trust in a location that's out of reach for the authorities it's even easier. It isn't like the FBI is busting down the doors of the encrypted HD ransomers.
Like I said, the best you can do is mixing and similar, but you know what criminals actually primarily use? Cash. Cash is fungible and untraceable. If you're going to do some hostage taking, you may as well do it for cash rather than bitcoin, the only potential benefit of bitcoin would be verification might be easier than cash.