I'm more on the fence about this reveal myself. I didn't think it was that well set up and we don't really know enough to see if it is well-executed.
I'm like "Okay, now what though?"
To be speak more specifically:
spoilers:end of spoilers
If anything this, to me, kind of weakens the interaction between the characters. Taking what was an interesting dynamic - the hero and his apparently increasingly violent and unstable mentor/father figure coming into conflict - and have now altered it to "your dad is a dark version of you," which is... not bad, but is different.
I felt like there was ground to be covered with Ziggy and Shiki as was still and this is just changing their relationship.
So... on the fence. Eden's Zero is still Mashima's best work since Groove Adventure Rave in my opinion, so I trust him to do something interesting with this but... I wouldn't have made this choice myself.