Undyne was already mad when she was taken away from her goal moments before being able to reach it. Plus, she couldn't use her magic here! And nobody seemed to have any answers! So when someone who looks to be a human launched himself at her, clearly trying to attack her, she was freakin' ANGRY.

She started running towards the human. She didn't know who this human was, all she knew was that he was attacking her.

She pulled back her fist and yelled, "NGAAAAAAAAAH!", as she tried to punch the human in the face.