You cannot kill a sleeping character. You should not try to find creative ways to kill a sleeping character.
If you encounter a sleeping character while RPing, for instance, breaking into someones cube or apartment, you should treat it as you would real life. Like they might wake up.
If you encounter a sleeping character on the street, one who you know is most definitely NOT a newbie, or if you frisk them and find sixteen sets of Xo5 battle armor, this player is PROBABLY experiencing connection trouble. You can drag them to a cube and leave them a note, treating it like they ICly passed out and you rescued them. They can then choose if they want to thank you. You can also xhelp and ask an admin to put them in a coffin due to them having connection issues.
If you encounter a sleeping character in a situation where you do not think they are experiencing connection issues (and feel free to xhelp and check with us) then you are free to rob them, move them, turn them over to the WJF, etc.
Sleeping is not a get out of jail free card. Your character can still be robbed, moved, accosted in other ways (like tattooed). Finding a 'safe' place to sleep is important.