I think that the weapons are hinting towards the fact that Nero was a human that could, for some odd reason, accept Dante's and Vergil's blood and thus, you have a stronger version of both of the brothers and a way to unlock the seal. The arm is a byproduct of the experimentation and it is in always in a "Devil Trigger" like state that requires some energy to use. Also, I believe that this is why he has the BLUE Rose and the RED Queen and also wears clothes that have both colors. Dante was outmatched in the opening fight which gives more substance to the mixing of blood. Other thing is that the thing Dante wants is the PERFECT amulet because it has the power to enhance Nero's power within him and allows the seals to be broken along with a carrier of the blood of Sparda.
Nelo Angelo (Demon Version of Vergil)=Blue
Dante's Sparda Devil Trigger (Demon Version of Dante)=Red
Does this make sense?
Nelo Angelo (Demon Version of Vergil)=Blue
Dante's Sparda Devil Trigger (Demon Version of Dante)=Red
Does this make sense?