How to kill someone in Minecraft using the console
/kill
Is not a OP command but a player command to commit suicide so it can only apply to the one who use it. (Source)
But... if your server is running on Bukkit, you could install Essentials plugin and the you'll be able to use the command
/kill <playername>
and a lot of other useful stuff that every server moderator should have. But if you are not on bukkit or any modded server then why don't you
/tp <playername> <yourname>
so the player will be teleported to you and then just kill him :D
One thing that you can do is simply do /tp <playername> x y z with a large negative y (and any x and z). They will quickly die of void damage even if they're already flying in Creative Mode. It has to be a large negative y, since void damage doesn't begin until -64 and flying Creative Mode players could just hover safely and tunnel back up. For example:
/tp <playername> 0 -1000 0
The only drawback is that the player's items will disappear entirely if you do this. However, depending on the circumstances, it sometimes is still good enough.
For Minecraft >=1.5:
You can put the "instant damage" potion effect on the player using the /effect command:
/effect <Player> 7 30 200
This gives "instant damage" effect (id 7) to <player> for 30 seconds1 with amplifier of 2002.
Since each level deals 3 hearts of damage, amplifier of 200 should deal 600 hearts of damage which should be enough to kill the player most of the time.
Exceptions where this method don't work: when the player is in creative mode, or the player has a really strong level 53 (not 5 to the power 3) regeneration resistance effect (e.g. via custom potions or /effect command, see below). It also doesn't work during a 3-second interval right after the player respawns from death (data obtained via experimenting).
Note that when the player has a level 5 resistance effect, the player is immune to most (if not all) damages. That means even falling into the void and/or executing /kill.
You can try the following commands (in client or SSP):
/effect <Player> 11 1000000 5
/kill
/tp <Player> ~0 -1000 ~0
After executing these commands you should still be alive but not dead.
Though, you can still "attempt" to remove the "resistance" effect by the following command before attempting to kill the player:
/effect <Player> 11 0
Sources: Minecraft Wiki - Commands and Status effect
Notes:
1. The duration time doesn't really matter, I choose 30 seconds simply because it is the default value.
2. Values above 252 doesn't work (possible bug?). For values above 248 the behaviour is a bit strange - there is a short delay.
3. From the Minecraft Wiki...?