Will mice leave if one dies?

Mice will not automatically leave your house just because one mouse dies, because their behavior is driven by food, warmth, and shelter, not by mourning a dead colony member. In fact, if one mouse dies in a trap and you leave it there, other mice may initially avoid the area for a short time because they can smell the dead mouse and may sense danger, but that avoidance is temporary and they will soon return if food and shelter are still available. If a mouse dies from poison inside a wall, other mice do not understand the cause and will continue to live in the same space, and you will have the added problem of odor from decomposition. Mice do have some ability to learn to avoid traps that have killed their companions, which is why pest experts recommend moving trap locations and changing bait types. The only things that truly make mice leave are removing what attracts them, which means sealing all entry holes larger than a quarter inch with steel wool and caulk, storing all food in airtight containers, fixing leaks, and removing nesting material. Killing one mouse without doing exclusion and sanitation will not solve the problem, and new mice will replace the old ones quickly due to rapid breeding.