There are any number of reasons someone turns to killing. Be it reasons they can control, or reasons they can't. One dead body, sad as it may be, does not a mass grave make.
The world is full of monsters, mind. Many a man has become one and there won't be an end to it till there's an end to man and beast alike. So, while you should always be careful, don't let this weigh on your mind more than need be. We've no details, no context. Murder isn't murder without motive, anything else is manslaughter and the reasons for that range from accident to punch-up gone wrong.
If anything, this incident might make you think twice about learning some self defence and knowledge of basic and improvised weapons. Nothing stops someone intending you harm like a broken off chair leg to the nose, after all. Sharp end first, of course.
Oi. I trust you're not too worried? If you need me, I'll actually carry my cellphone about. And turn it on, no less.