[To be fair to Sebastian and his murderous fantasies, he has a healthy amount of self-restraint. Maybe he didn't always quite so much; maybe Time knew him back when he was younger and more impulsive, but that's what age does to you, isn't it?]
It is a butler's responsibility to protect his master from all that might befall him, and to ensure he is in optimal condition, physically and mentally, at all times. [Because that's all Sebastian is to Ciel, obviously. A completely ordinary butler. Just a little roleplaying to pass the time.]
To fail at such a fundamental task is to break the butler's aesthetic. ["I'm upSET because this doesn't make me look good."]