Joaquin Dead Phoenix (title not relevant, just wanted to make that pun)

When I watched the Walking Dead pilot the other night, I have to say I was intrigued. Not because of all the symbolic meanings taken on by the various shots and role changes (that was just an added benefit), but by the changes made to the script. Such as when Rick wakes up in the hospital, and he starts wandering around. When he comes across the barricaded door with "Don't open. Dead inside," written on it and he sees set of hands trying to open it from the other side. He just sort of avoids it, even there is no reason, at this point, for him to realize that it's a zombie apocalypse. Then later when he just casually caps the rookie cop only to have that emotional mercy killing of the bisected zombie a minute later. All I could ask was "What's going on here?". I realize that some changes have to be made when transitioning from a written/drawn medium to television, but did they have to make such strange changes to Rick's character? I don't even mind the changes to Morgan, even if he was turned into a more practical survivalist character. As a police officer, I wouldn't be surprised if Rick has had to take a life before, but why exactly was he so easily able to kill one of his fellow officers only to start breaking down when one stranger has turned into a zombie and lost their legs? Did he just not like him and was thinking, "Meh, he was kind of an idiot anyway."? What made the crippled zombie so special? Why did he have to track it down just so he could end its miserable existence? I don't know, but it keeps me up at night.... Well, that and wondering if penguins have knees.