Can I just say? That heartbroken expression captures the feeling of getting dunked really well. That feeling of betrayal. That moment of disbelief. You can't understand why he did it, but he did. Despite all the fun times and shenanigans you two went through, despite all the jokes and pranks he knows he's pulled in you, he still can't see you, the player, anymore. All he sees is the monster, coiled around your neck. So, you're just left there, wishing you could go back. Wishing you could laugh with him once more. Wishing you could have him prank you one more time. Well, he's giving you a way out, he's giving you your wish and you have to decide whether to use it or not.

It hurts even more knowing you can't do anything to explain why you're killing everyone he's ever cared about.

Additionally, this gives us an idea of how it must feel for Sans to do this. Though he does say that infamous line with a tone of playfulness, it may just be part of his fascade. After all, he does want to consider you as a friend as evident by the end of his dialogue. So it must hurt him deeply to have to kill you in order to kill the monster that plagues you. Cause in the end, all he wants to do is protect his friends, including you, the player, even if he has to take your life.

Anyway, sorry for rambling unnecessarily about basketball skills. Here, have a gif.

Keep in mind this is supposed to be playing.