The last fight to the game Undertale was a memorable one for me, and I never really understood why; that was until I rewatched the last fight with Asriel a while back that I began to understand and put into words just exactly what makes it such a bittersweet moment.
The reason that Asriel is fighting so hard to defeat you, is because he understands that he is trapped within a video game despite every action that you take with the characters provided have very real effect to the monsters lives. It genuinely affects them. When he died and became that flower; doomed to repeat game after game, it wasn't until the player (Frisk) came into the picture and provided something new. The reason he is trying to defeat you so desperately is because he doesn't want the game to end. He doesn't want you as the player to be bored and leave once you are done.
I also theorize that the other souls, as well as Chara were actually humans playing this very same game that Asriel managed to figure out he was apart of. When Chara as well as the others had died, the person playing as them simply stopped playing the game. https://youtu.be/RH05QKkk6Q8?t=1234