Is it ok to write a letter using cut out magazine letters?

To a friend for a ha-ha thing, but in any other situation, it will just be ignored.

Sure if you’re a serial killer.

It is not alright.

I don't believe there is a law against it, so you could do.  

It's standard practice if you are a kidnapper on television series.    

So if your filming a television show and you are playing a kidnapper, then 

it is what you would normally do.  

Otherwise, I would not recommend it, because people might think you 

want to grow up to be a kidnapper or blackmailer.

Only if it's a poison pen letter.

Only if you're writing a ransom note.