The point to fighting back was to help save your friends, Sans. If you hadn’t held control for long enough to warn Toriel of Chara’s attack she would’ve easily died. And if you hadn’t made it so hard for Chara to harm others then they would’ve killed others as well. It may seem futile to fight back, but it has still been helping. Hold on.

Sans, don’t give up! Your friends are always beside you! And I am too. We all are. You can overcome this, we believe in you! Your friends are trying to save you right now, so don’t give up! Even if you’ve been through worse… even if you think there’s no point in trying anymore… and even if you think it’s hopeless. Because… there will be a light at the end of the tunnel. Oh, and also, Chara is trying to bargain you for Frisk. But if you overcome it, then maybe things could be different.

Sans, the point was that if you fight back, the world would be much safer. IF you fight back, Chara wouldn’t kill. So many innocent lives would be saved. There is always a point to fight back sans, you just have to find it.