The game is more about making you feel differently.
When I first played episode 2, I tried to save Larry, but then Kenny crushed his head. I was so shocked and rather angry at Kenny. When he asked me what I thought of what happened, I stayed silent. I honestly had no words for him, and my opinion of him and the way I felt about him changed from there on forward.
Someone who decided to kill Larry would feel very different to the way I felt. They probably felt a little guilty for acting so soon, for putting Lilly (and Clementine to a lesser, but nonetheless still powerful degree) through it, but they probably also felt like they were saving everyone's lives and protecting the group, and that it had to be done.
These are two very different outcomes, and this doesn't even bring into it the way the in-game consequences change.
What you're asking for just isn't realistic. The whole point of fighting with or talking down Kenny was so you could stop the train. You want to know what happens when you don't stop the train? Duck kills everyone. You can see that for yourself.
Sure, some of the choices seem like they could have played out differently, or had a greater impact on the story, but it's not really within Telltale's power on such a tight schedule and for such a low price.
No game that currently exists has truly branching choices that affect the way the story plays out (well, I've heard things about The Witcher 2 doing something to that effect, but I'll have to play it before I can say anything). At this point in time it just isn't economic or efficient.
It'd be awesome, sure, but it's probably not going to happen for a little while.