Undertale - Yoko Taro's thoughts on Undertale (tweeted by Toby)

It's not, he's just a baddy who kicks your ass hard if you voluntarily make the wrong decision. Undertale has plenty of other cool stuff, but he achieved cult status for some reasons for being legendarily difficult.

He kinda is to be honest, but he's designed to be. Basically, he's here to stop you, and because the game is meta as fuck and he knows he's facing a player that can try an infinite number of times, the way he does this is to kill you endlessly until you give up or choose another path.

The optimal state for gaming, if you ask me.