Maul would mostly kill Satine yes, but he was not going to do it that soon if Obi Wan didn't enrage him. Remember he was torturing her to let Obi Wan go to the Dark Side before he was enraged, so he would not kill Satine soon because he needed her as a tool to let Obi Wan succumb. In this kind of situation the best choice is to buy more time, as long as Satine is alive there would be chance for others to rescue her. It's just like someone took a hostage, he said he will kill her and he does hate her, but is it wise enrage him? No. It's a hard job but he got no other choice since it could be solved better if he didn't refuse Anakin's help.