Well, this is one of the inconsistancies I've seen in the old cannon. Before LoI came out, Dracula was simply Vlad the Impaler who became a vampire and use dark magic to amass an legion of demons to terrorize humans. The strength of the dark magic and demons was reenforced by the darkness in the hearts of humans. My guess is that Chaos, as it was called in AoS, is just the physical manifestation of the evil. This evil seems to be destruction incarnate is by its very nature is chaotic.
With the release of LoI, things likely changed. Instead of using the darkness in the hearts of humans, he uses the crimson stone to increase his power. Remember, the crimson stone turns the souls of vampires into power for its master. Seeing how Mathias/Dracula is the master of the crimson stone and it is likely the Belmonts were hunting vampires for the 300+ years between LoI and CV3, it is likely that Death would collect the souls of the vampires killed by the Belmonts and give them to Mathias. With that understanding, he never stopped manipulating Leon and Leon's descendants. He simply indirectly used them to amass power for himself.
Keep in mind that this is all theoretical based on my understanding.
As for Gabriel in LoS, he stole the power of the Forgotten One and then killed him with his own power.