Who invented heavy metal?
The phrase heavy metal (in music) was literally invented by Steppenwolf - Born to be Wild (1968).
For opinion on the question, read on.
The Kinks' You Really Got Me (1964) was probably the first example of a metal riff (where Davies reportedly slashed his amp with a knife to get distortion), and Hendrix pioneered the guitar histrionics that characterise a lot of metal. Most people say that 'metal' itself was created by Black Sabbath (1968) though; riffs that were no longer so bluesy, the occult theme, the heaviness, the power chords, the pessimistic outlook. It's possible to trace most aspects of metal back to Black Sabbath.
Other bands such as Blue Cheer (1967) and High Tide (1969) are also credited with inventing metal.
I'd also like to shout out Gustav Holst (1916) and Jeff Beck, who is credited with being the first guitarist to create a way to control guitar distortion (circa 1966) and, (just my opinion) recorded the first metal-style guitar solo on Heart Full of Soul with the Yardbirds, released in June 1965.
Heavy Metal was originally coming from Rock. Because Rock isn't that hard like Heavy Metal and Heavy Metal was invented, it shows the difference between both music genres. Heavy Metal must have hard beats as same as country music needs accustic guitars and heartbroken lyrics. Same as techno needs infinite amounts of the same beats with few breaks, but the rythm is the same all the time in a loop.
This is the same kind of question why the neanderthal man invented the fire - because humankind is curious and loves to experiment. Heavy Metal was developed because the former genre Rock wasn't the kind of thing that people liked or atleast they wanted something heavier - Black Sabbath, one of the first Heavy Metal Bands developed Heavy Metal from Rock and opened a new age of music.
It has different reasons. For some have bad experience with people who listen to heavy metal - the parents might even listen to heavy metal and had bad experience. However, parents think that heavy metal has a bad influence to their childs because most of heavy metal bands are considered rebellious or exalt violence.