Once the Hunger started, Monster Kid’s parents almost immediately started to feel the effects. With the little sanity they had left, they forced Monster Kid and his sister to lock themselves in their bedroom, and to only come out once everything was quiet. Two days passed, and they could hear the screeches of their parents finally die down. Once it was gone completely, they came out, finding their parents dead, with their mangled bodies in the center of the living room. After weeks of feeding off their parents and hiding inside, they ran out of food and grew hungrier and hungrier. Soon, Monster Kid was consumed by the Hunger, and turned on his sister, devouring her almost whole. This ate up the last of his sanity. He painstakingly stitched the last thing that was left of his sister, which is her arm, to his armless body. The arm doesn’t do anything. It is stuck in the same position and just dangles loosely from his scales. The reason he did it is unknown, through it’s best to just chalk it down as insanity.
Monster Kid now stalks upper Snowdin and in the wood lines around the area. His tail spikes grew sharp and long, and now serve as his weapon of choice. Once he has his sights on a victim, he emits a loud, screeching roar and darts forward, swinging his tail and impaling his prey on it. He would sometimes leave them there, squirming until they bleed out.
Those were the lucky ones.