Go to this link for descriptions of how to get all of the achievements: https://www.yekbot.com/the-shapeshifting-detective-achievement-guide/

In the linked guide the description for this achievement is worded as if you can be killed during any of the killer confrontations. For this achievement you need Rayne to be the killer and you need to NOT pick him when Dupont asks you who you think the killer is. This will lead you into a confrontation with him. You will need to shapeshift into Rayne during the confrontation when given the option.

The killer is randomly selected in each playthrough but there are some hints during the game that will give you an idea of who the killer is fairly early on. If Violet denies knowing where Dorota lives she's the killer so quit the run. From then on there are clues based on conversations you can have with Rayne as Lexi. Bring up the topic of him praying when you have the opportunity. Also ask Bronwyn about the gold coin when playing as Sam. Thanks to Steam user Varstahl for this information.

Please rate the guide if it helped and post any comments if you think of any clarifications I should add.

Thanks for reading, my dudes.