In this scenario, Dreven did what he did on the ship out of necessity. His actions would have caused him to go into a blood rage because he felt he needed to survive, and Sylvanas did mention that the crew was expendable. We give him emotion by having it bother him after the fact, and applying this to his people.
His attitude in the scenario is also meant to show his status as a Blood Prince. One that does give respect, but isn’t afraid to back down and bare his fangs should he feel threatened. He leads his people, and they have been through a lot. The San’layn would see how similar she is to the Lich King, and it causes a distrust. He accepts Sylvanas’ promise not because he believes he, but he wanted to hear her actually say it.
Sylvanas also can’t just outright kill him out of rage. Not just because it would cause problems with the Horde, and it would, or that she wants the San’layn within the ranks, which she does akin to how she wants to decimate Stormwind to add to her “people” so to speak, but she knows he is right. If she kills him, he wins, and she knows it. She would get no satisfaction, and this drives her crazy.
Dreven: Thats the thing with you, isn’t it? You got a taste of it when you were under his command. And now, you just let go of your feelings, your memories, your true people, and for what? Toys you call an army. Toys that you just have no problems breaking every time you have a hissy fit and want things to go your way. You like the power, don’t you? It’s funny, I’ve lived for so long under him, and I remember who I am. You just remember what you want…and that is why, my dear warchief, you will fall one day. Just. Like. HIM.