If using : /gamerule doMobSpawning false, then it doesn't make any changes unless you type in the command : /kill @e. Anyways, I tried using .kill @e[type=!player], it killed me.

Actually it's much more beneficial to do this:

• /kill @e[type=!player]
  

Otherwise you'd be killing yourself too. Also: if you immediately teleport to a location after having killed all the mobs then chances are high that you nullified the effects of killing those mobs because they normally auto-generate, so every time you teleport to a new area ("chunk") then chances are high you'll get new mobs.

If you don't want that use: /gamerule doMobSpawning false.