I've noticed many fishy and illogical things and events taking place after the dinner, but one in particular has been bugging me:
How did Mark end up as a walker (or should I say crawler) upstairs after the dinner when you confront Brenda?
Everyone will remember Mark crawling downstairs and in front of the dining room before Lee gets knocked out. Why on earth would the St. Johns carry him back upstairs and leave what remained of his body intact? He was obviously either dead or dying, so they would have a crawling zombie upstairs while busy dealing with the people in Lee's group. It was a completely unnecessary risk - they could just have bashed Mark's head in, or thrown him on the other side of the fence (although he might've been able to crawl underneath the fence without getting electrocuted).
Of course it was quite heartwarming to see Mark helping out the group even after his death, but it still seems ridiculous.