or it will be possible with some mod in future ?

anyone know ?

It's not possible in the vanilla version, but with mods it's already possible. If you go to TES nexus you'll find mods that make all NPC's killable. Be warned though. They could break your game if one of those you kill is involved in quest you take afterwards.

but it wont break the game if i first finish my main quest and then load the mod and start killing everybody ?

It won't break the main quest since you will have completed it, but there's no telling if it might break other quests you haven't done yet. Another way, if you only want to kill a specific npc, is to find an npc's ID and disable the essential tag yurself through the console. That's what the mod does anyway, it just does it for all the npc's. (apparently it might not work on everyone. Some comments said there are people in the companion's guild that weren't affected. It's still the best one can get without the construction set.)

Here's the mod.
http://www.skyrimnexus.com/downloads/file.php?id=666

The mod is basically a text file with the command "setessential *npc-id* 0" repeated for every essential npc. You simply put this in yout skyrim folder, run, open the console and type "bat noessnpc".

thank you!