So how's that your problem? She cheats on him, he should be pissed at her, not you. Especially given that you didn't know she was attached to begin with. If the boyfriend is the unreasonable type, that's what guns are for. (oh, wait, you live in Pussyland where you're not allowed to have guns, don't you?
)
Probably legal with a licence, But we have a law that helps us when you act in fear of your life

You would get more time in jail for 1kg Cocaine then you would for murder in sweden.
One guy who killed 3 people because he wanted to steal a damn bike and cofessed to everything was realeased a while ago arfter serving I think about 15 yr in prison..
You dont hear about that stuff in USA lol.
Fucking retarded prison system in sweden.