You also need more than 150 humanoid kills. Kobolds and bandits count.
Alternatively, you can also get full points if you get 150+ renown, 20.000+ silver, LESS than 3 friends and having science as your highest skill.
If I remember correctly, once you get 150 humanoid kills, it will override any other skill you have and give you the assassin ending, so you can have for example farming lvl 500 and 151 humanoid kills and you will still get the assassin ending.