Make the first sentence usable
Lead with the reason for writing so the recipient does not have to decode a long emotional introduction. For a message about how to apologize for not returning a neighbor's tool, use this factual base: state the date, place, property, noise, pet, or shared-space impact without arguing about the neighbor's character.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. Offer cleanup, replacement, access, a schedule, or another practical remedy you can complete. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respect the boundary already visible
Do not threaten, shame the neighbor publicly, or involve the whole building in a private issue. Use property management, local authorities, or emergency services for genuine danger rather than confronting someone unsafely. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.