Aokiji has his own sense of justice. Compassion is irrelevant to him. He bases his decisions on what he thinks is right and wrong.

Kizaru is indeed mischievous, doesn't seem to care too much about the damage he causes.

Akainu we kind of know from Robin's flashback. He ordered the ship full of civilians to be blown up. Clearly, he's the most thorough and ruthless of the three.