Imm not saying imm right we can all belevee what we want to , imm going to say a couple theroyss one is mine and the rest are the most common.
Now I dontt have proof but.... Maybe herobrine was a kid just like us or teen and after he got home from school he started to play minecraft. Heringg a thud he turned to see a man in a mask he screamed and those were him last moments. Well that's one of my theroyss.
Another theroy i have seen is herobrine is a Swedish man thatss trapedd in minecraft. Ivee seen this teroyy about 6 times....
steves brother , herobrine being greedy went for dimondss above lava and Steve the younger brother was careless and acedently shoved herobrine off into lava
feel free to edit with more theroyss