[Chapter 1 Spoilers] Found a really easy way to kill the Thieves' Guild.

[Crestfa11en]

The first guy, Jasper, can be killed normally since he only has rookie 1h. Just swing forward and parry instantly after, repeat this until he's dead. The next one, Ramirez can be killed very easily. Step 1: Swing at him one time. Step 2: Sheathe weapon and talk to him. Step 3: Repeat. He will never become hostile towards you and you can kill him very easily this way.
You can use this same tactic on the leader, but it takes a bit longer since after you hit her, she likes to clip into the book she's reading and you won't be able to talk to her. Just wait until she puts her sword away on her own and swing at her again. You will easily be able to get 2 Master Swords and join the Militia quickly as well with this method.

[podróżnik jezusa]

aye mayne, just slap some transformation scroll on yo ass and bite the ♥♥♥♥ out of them with a dragon snapper.

[Oneeyed]

yeah you can do all of this but then you lose out on the quests they give you later.