I think they tried to make the opposite of Kira Jesus Yamato by making a lead fueled by hatred who does not shy from war.
There was also a lot of experimentation with hero / anti-hero + siding with the devil but those traits just ended up clashing against the typical shounen lead traits.
Meaning the character had all the faults found in common shounen heroes, but also, due to the experimentation, wasn't working for what is perceived as 'good' to excuse the faults.
(i.e. the difference between Shinn and Naruto is that Naruto at least works for global 'good')
Another big "faux pas" is him being disrespectful to EVERYONE
from the first serie. That is plain character suicide considering GSD exists because of GS.
A good example of what happens when you piss on your predecessor would be Jay Wilson.
Still, at least one good thing came out of Shinn:
On August 8, 2011, Shinn's voice actor, Kenichi Suzumura, got married to Maaya Sakamoto, none other than Lunamaria Hawke's voice actress. dawwwwwwwww