AITA for handing in my notice at work?

I work as an administrator for medium sized company for about two years. My boss is really difficult to work with and has made several unprofessional comments about my clothing, weight, a sudden family loss, my anxiety disorder, and my home. In addition to this, he treats me as his personal assistant, asking me to go and get wine for him for a dinner party he was hosting at home, telling me to add heavy items to my luggage when we were away at a conference so he wouldn’t have to check a bag on the plane but I would instead, and laughing at my previous experience (which is actually good), because I’m in a lower position to him.

I took a step down to come here, as I have been attending graduate school at night for the last 2 years and it suited me better to work 9-5. I have just completed my MBA so I asked for a raise and was told no, there is no chance of me ever getting a raise or a promotion here. Then, I started looking outside of the company, and was offered a job as a department manager for a lot more money than I currently earn.

Last week, I handed in my notice and was a bit shocked at how it was received. The MD was extremely nice about it, saying he was sad to see me go, wishing me the best of luck etc, but my manager barely said anything. He said thanks, and ignored me and started typing. When I tried to discuss it, he ignored me, then left for the weekend, walking right past me without a word. I then received several very rude and demanding emails about needing things to be done ASAP (all at about 4.30pm on Friday). I arranged for these things to be done, but some required confirmation from him which he ignored.

Since then he has continued to be extremely petty, walking past my desk and greeting everyone there by name and excluding me, refusing to speak to me except via email, admonishing me for being 10 mins late due to a crash causing traffic (which made everyone late but only I was reprimanded), and I have just found out that he lied about all the ‘urgent’ things I had to stay late to do on Friday. In addition to this, two of my colleagues are now treating me like a pariah since they had a meeting with him this morning, although the rest of the team are fantastic as ever. I have also been removed from the reports and emails I need, so I can't do my job, which I only found out when I called an external supplier wanting to know why I hadn't received a confirmation from them. It was upsetting and humiliating.

I gave 3 months’ notice as per my contract, and I have drawn up SOPs and training manuals for whoever replaces me but I feel like I’ve done something horrible to him because of how I’m being treated. My mother said it’s probably because I didn’t tell them I had begun to play outside the organisation, but I feel like that’s normal. I didn’t even accept the other position until after I had handed in my notice, as I wanted to give them a chance to perhaps counter, but instead I’ve been subjected to a lot of pettiness. AMITA?