If by utopian society, you mean a society in which you wouldn't have the ability to choose to do evil, then perhaps Kira would be successful.
However, I far from consider this a utopia. There is no choice, only a blind following from fear of death. Is it really being a good person if you are just too scared for your own life to do wrong? One could still be harboring evil thoughts and of course there is the paradox of "Killing is wrong so I will kill to right the world."
More than likely, I would eventually stand up to Kira, but, being nowhere near the mental prowess of L, would quickly be killed. Darn.