I know that many fans think it is insulting that they changed what they did. Not just the hair, but the setting, Dante as a character, the weapons and such. That instead of following up on the original series, leaving so many unanswered questions and plotholes behind, Capcom decided to make a do-over.
I also know that there are fans who find it insulting that Capcom and Ninja Theory haven't been very empathic with the fans and have instead just said "When the game is out, the fans will like it".
I generally believe that it is lack of communication that is behind this. Haters are gonna hate, no one can change that, but maybe there wouldn't be so many haters if Capcom had made it clear from the start why they are doing this. If Capcom have said something like "We wish to make a Devil May Cry reboot which appeals to more western gamers, because we feel like we need to expand the fanbase and we do not believe this is possible with the current series." or whatever their reason for making this new Devil May Cry is. If they had done so, then the fans would have something to relate to, instead of just feeling left behind.
The fans of the original series have been neglected, there is no denying that, and it has resulted in the fact that there are so many angry fans who will find any excuse to throw hate at them. The overall message given to them is "This is our franchise, we can do with it what we like and you have no saying in this.", and if you are a very dedicated fan to the original series, then you do not wish to hear that your dedication is worth zero! It's like suddenly realising that the one you love don't care about you one bit. It has made the fans bitter and emotional and that means they will easily feel insulted about the smallest, insignificant things, because to them it IS significant and definately not small.
That's the impression I've gotten at least.