Kuzan's justice changed, he used to have "Burning Justice" and he changed because Akainu's genocide at Ohara, so of fucking course he would oppose the same guy getting to that position.
No it's not, had Kizaru wanted he would also oppose either of them, he just doesn't care.
Kizaru is as capable and strong as Akainu and Kuzan, in fact Kizaru had the same epithet as Akainu when he joined the Navy: