Yeah, it wasn't like he went into a homocidal "I'm evil, let's kill!" mode when he killed Sifo-Dyas. Aside from the pathological and psychological games that he plays, Dooku isn't outright maniacal or venomous to people. He killef Sifo-Dyas because Sifo-Dyas had predicted the Dark Side's coming rise - so Sidious ordered Dooku to kill him to prevent the Council from learning, as well as - yes - an act that would cement Dooku under his thrall.