It's a free game if you know where to look, modding is easy and fun! It's a classic from 1993.
also, if you like rage games... someone here might get mad at me lol, he didn't like it... i made a garbage doom wad (a wad is a mod - game data files are stored in a single .wad file - it stands for Where is All the Data) and it's a giant maze with tricky unfair combat + a few people liked it surprisingly; others think they are crazy.