You killed an other player and now you are a murder. U can get rid of that by getting killed by other players. A few kills and you will be ok. Or you can simply wait, but u will keep losing exp and hp while under murder status.
Notes:
- You can lose your items, while they kill you. Put everything in storage first.
- The skull icon will remain on your screen for a few days after you got rid of the murder status, don't worry about it.
- You can't pay in Hunter Guild to get rid of it, because only wanted thiefs can do it.