Alone with
The murder condition: the victim and exactly one other suspect share a region with no other suspects present.
Explanation
This is the core murder logic of Murdoku. After placing all suspects, find the region containing the victim. If exactly one other suspect is also in that region, that suspect is the murderer. If zero or more than one other suspect shares the region, your placements are wrong.
Example
The victim is in the Library. Max is also in the Library, and no other suspect is. Max is alone with the victim → Max is the murderer.
Common Mistake
Players sometimes check for 'alone with' before all suspects are placed. The murder condition is only valid after EVERY suspect has a confirmed position.