Agreed. Given that Marcus is supportive of Kait’s desire to find answers, I suspect he is totally aware of this. And besides, JD also went after Marcus after he was taken by the Snatcher. Alright, the context is a bit different and they needed Marcus to help guide them to find out what the Swarm where and to eventually find Reyna, but as covered by the conversation between JD and Del just before they got on the ore transporter into the hive, Del and Kait trusted him and went along with it.
I do get a sense of double standards here by people who for various reasons, just dislike Kait.