Is it possible to kill characters that go unconcious when you reduce their health all the way? I want to start a new character and try to kill everyone in in the game..

Not without mods or the console. Essential NPCs have a second tag on them. This mod removes only the Essential tags from all NPCs, and this one removes the Respawn tags as well.

Happy slaughter. Hope you don't get bored with it too quickly.

Thanks dude

If you have any problems such as a mod conflict, you can try Console commands. Here is what the UESP Wiki says, but there's a warning at the end:

SetEssential BaseID #
(Only works with NPC and creature IDs.)

Makes the NPC or creature belonging to the ID essential or non-essential depending on the number used (1 for essential, 0 for non-essential). Be careful if you use this on a creature as it will affect every creature of that name.
Ex.: SetEssential A288 0; removes Countess Millona Umbranox's essential flag.

Links: Lists of NPC IDs: Named NPCs - Generic NPCs; Lists of creature IDs: Animals - Horses - Undead - Goblins - Monsters - Daedra - Dremora.

Other applications: N/A

Notes: Be careful with this one as you can break quests if you make the wrong NPC non-essential at the wrong time, but you can also protect a quest by making a non-essential NPC essential.

SetEssential <Base ID>