Okay, so basically, the game revolves around two concepts:

-Unlike most RPGs where killing enemies is an average thing, Undertale gives you a MERCY option, in which you can spare your enemies instead. Nobody has to die! It's about what you think is morally right

-There is also the fact that no matter what you do, this game will remember your EVERY action. And to find out what I mean by that, you just have to play it yourself ^3^