If you combine RogueKiller with another Antivirus programme on your system, you will have false positives on the AV components that seem like a rootkit to Roguekiller and vice versa. This is quite normal, since Antivirus software needs to deeply ingrain with the operating system, intercept certain API calls and be robust against attempted disabling or deletion by malware. So it uses some techniques that are also often used by malware and look suspicious to a rootkit scanner. Furthermore, the signatures used by Roguekiller and Bitdefender trigger detections on each other as well.
Unless you feel confident to be able to distinguish between real threats and AV components, I do not recommend mixing them. You might end up disabling or damaging vital Bitdefender components without noticing it.
(Edit note: I had to revise my first statement as it seems RogueKiller has gotten some further development and was now published as a full consumer anti-malware engine.)