Based on a real life serial killer

[wondermanrules]

The movie is British and based on a true story from what I recall. I mostly recall the killer ran around wearing an all black military type outfit with a black ski mask and was robbing and killing shop keepers with a shotgun.

I remember early in the film it shows him training himself by filling a backpack with rocks and running across fields and hills to build up his strength.

At some point he kidnaps a girl and buries her underground in a failed ransom attempt.  I only recall things go horribly wrong, he gets captured by the cops and by the time they find the girl he kidnapped she's dead and I think he gets the death penalty.

I'm unsure when it was made. Possibly the 80s or 70s but I definitely remember it said the events in the film were based on a true story.

[Bargle5]

I think this is your movie. "The Black Panther" based on the crimes of Donald Neilson.

http://www.imdb.com/title/tt0075764/combined

[wondermanrules]

Yes! That's it, thank you! 

[Bargle5]

You're welcome.