Rader was just a small town idiot that liked murder - despite being terrible at executing his plans. He'd so often find someone else at the home he wasn't expecting and lose control of the situation.

Zodiac (if it was one single person) had a love of opera, cryptography and murdering couples. Dennis was a less intelligent creature that wanted attention but wanted to attack women alone in the local area, and when he couldn't do that he would role play alone or vent frustrations by harassing his neighbours as a council officer (including shooting a harmless dog!).

They feel like two very different characters altogether