Several of the mystery writers I know really have a lot of fun with their victim. Sometimes an author purposefully makes the victim similar to someone he's had a run-in with or someone they didn't like in high school. There are plenty of unpleasant people out there and it's easy to make an amalgam of them to form your perfect murder victim.
The victim needs to be someone murderable (I know, it's a made-up word). In other words, someone who has a few enemies. Actually, this is a rule that I'm playing with a little for the book I'm writing now. I start the book with everyone having warm fuzzy feelings for the future victim, but then we start finding out a little bit more about her. Then we know why several people would like to bump her off. Otherwise, why on earth would your victim legitimately be murdered? Unless you're writing a thriller with a serial killer who just kills random people.
I've read books before where the victim is dead on page one. Later on, we find out more about the victim through flashbacks (not my favorite device) or through interviewing people who knew him. This can be really interesting, if done right. I found that I was very curious to find out more about the victim and having the information doled out in tantalizing bits and pieces held my interest.
I've also read books where the reader was fully introduced and vested in the victim before the murder. This works, too. The reader has gotten to know the victim and has more of a personal interest in who killed him.
How gruesome is the murder of your victim? This depends on the type of book you're writing. If you're writing a cozy mystery, the murder will probably take place off-stage and you won't provide your reader with many gory details. If you're writing a police procedural or a thriller, you can usually get away with a lot more. But keep in mind that if a book gets too graphic, you could lose some readers.