I have killed one inquisitor and my Assassin had a 95% chance. That was the highest I had ever seen, so I had to pull the trigger. I have also seen as high as 72% on inquisitors, but felt the risk was still too great. I have killed at least a baker's dozen family members, many of which had high success percentages. I have also had a lot of assassins die, but no more than in Rome.