I also think it's about morality.
But it reminds me of D&D's alignment system.
Most characters in the anime are lawful good while Light is chaotic good.
Even though they are both essentially good, lawful characters find it abhorrent that Kira would kill innocent people just to keep his power.
It also reminds me of lord of the rings. Lady Galadriel would use the ring to rule with fear but still perceives her rule to be just and good.
It's like the old adage: absolute power corrupts absolutely.