Sure, Alex did feel sad and guilty about what happened, but that wasn't what was on his mind. The research that they had found that day did say that it was a temporary cure, but it also mentioned that there would still be symptoms of the Croatoan in the host.
The story that he told Aidan was a much friendlier version of what really happened. There was blood everywhere and pure cannibal behavior. Screams could be heard from everywhere and you could only watch as your friends got torn to pieces and turned on you, only thought in their mind was to eat, eat you! It was really a hundred times worse than how he told Aidan.
Alex ran to the police station, it was the only safe place. He had searched around earlier and found a four walled steal cell in the basement of the station. Here he would be safe. He locked the cell gate behind him and then went into the steel room. That door he locked as well. He wasn't trying to keep anything out, he was trying to keep it in. Croatoan, even with the plant's effects still had it's time to reek havoc.
Alex threw his top and shoes outside of the door, he then prepared himself for the hunger. He used the cell's chains and cuffs to lock himself and make sure he couldn't get out. The hunger always came slowly. First he would have an enormous pain in his gut, then his jaw would crack and shift along with some other bones in his body. After that pain stopped he'd get an unbelievable migraine and come out of it as something else. Pure hunger. At this moment he
is a croat.
The hunger is so intense that he even consumes himself. His arms are torn up, and body scratched into deep cuts. It only lasts ten minutes, but it's three days of hell.
When he comes to, he is covered in blood and fresh wounds. Quickly he must clean up before Aidan notices.