A really shitty kind of music that's bastardized the name of hardcore and attracted legions of posers who basically stole the fashion from earlier punk rock, hardcore, and emo, overdid it to the point of looking really lame and deprived of attention as well as listening to bands that sonically have absolutely nothing to do with where their fashion comes from. So basically these kids have absolute shit for taste in music but enjoy flamboyantly dressing up in a desperate plea for attention, and are idiots musically and otherwise.
Deathcore was spawned basically from metalcore as it was becoming more overplayed and gaining more mainstream popularity. It was around this time that probably some ugly idiots who happened to be at ozzfest saw some metalcore bands playing and conjured up the flawed idea that combining shitty death metal and metalcore as well as dressing up could be "cool" or "cutting-edge".
Also most death metal sucks to begin with, listened to mostly by ugly dudes with long hair who enjoy playing dungeons and dragons and not showering.
Nonetheless deathcore fans are generally posers to both metal(because of the way they dress) and hardcore(because of having no real knowledge of the genre as well as punk rock, DIY, independent media, etc.).
The tail end of metalcore and deathcore mark a sad milestone for the hardcore genre, as this is the first time within almost 30 years of history of the genre that it is being bastardized and exploited in such a way, this has happened to metal numerous times throughout the years but only recently for hardcore.
The worst kind of deathcore people are the girls, these stupid misguided teenagers who've probably been molested by family members growing up or something see listening to shitty music as a gateway to dressing up, being slutty, and gaining attention. These girls have the intelligence of a brick wall and have no exposure or understanding of actual good music.
On a final note many of these bands find it necessary to down-tune their guitars in much the same way as nu-metal.
Deathcore: Suicide Silence, Job For a Cowboy, Whitechapel, The Acacia Strain, Through The Eyes of The Dead, Animosity, I could keep going on all these bands suck and are devoid of any real passion.