Okay I'm seeing a lot of posts talking about how they don't understand where Thomas is coming from on the whole Alfred/valet issue. I think it's pretty clear. Thomas had to go through so much scheming, failed endeavours, evil plots, etc. and had to wait years (which included a bloody war) to get promoted to valet. Now Alfred just waltzes in and because he's related to O'Brien the two of them think he should get to be a valet right away. And on top of that O'Brien wants Thomas to teach him everything he knows!
Imagine if you had a job and worked for years to get a promotion and some new guy came in and immediately got the position you spent so much time and effort coveting. Not only that, you're expected to train this person at no increased pay. I'd be pissed too. In fact I had a similar experience at an old job and I reacted much like Thomas. I was livid.
Let's face it, Thomas will always be a callous asshole, but his giving Alfred a bit of a hard time is the least of his offenses. And O'Brien and Alfred definitely know how to bite back.
That said, let me also say that the Thomas/O'Brien smoking circle is my favorite thing about the show and I wish they were friends again. But I definitely understand the views of both characters on the matter.