不倶戴天の敵と言われるほど、どうしてそんなに嫌われたかなあ。

、。

I was hated so much they called me a mortal enemy — what did I do to deserve that?

Literal

Irreconcilable enemy [quotative-と] said extent, why that-much was-hated [wonder-かなあ].

不倶戴天の敵 is a dramatic 四字熟語: 'an enemy with whom one cannot share the same sky' — a mortal, irreconcilable foe. ~と言われるほど = 'to the extent of being called.' かなあ expresses puzzlement with a trailing, self-directed wonder.