Is there a difference between 絶える & 死に絶える? 絶える already means to die out/to become extinct, so is 死 necessary?
絶える has a number of meanings, but it primarily means "to cease" (e.g., 仕送りが絶える). To express death using 絶える, you usually have to say 命が絶える. 絶える may mean "to die" by itself in rare poetic/literary contexts with the aid of the context, but you almost never have to do this unless you want to be a novelist or something. (By the way, to say "to die" poetically, 果てる is much more common.)
In addition, 絶える is about the life of one person, whereas 死に絶える is always about extinction or mass murdering.