1. Peaceful, placid, serene, tranquil refer to what is characterized by lack of strife or agitation. Peaceful today is rarely applied to persons; it refers to situations, scenes, and activities free of disturbances or, occasionally, of warfare: a peaceful life. Placid, serene, tranquil are used mainly of persons; when used of things (usually elements of nature) there is a touch of personification. Placid suggests an unruffled calm that verges on complacency: a placid disposition; a placid stream. Serene is a somewhat nobler word; when used of persons it suggests dignity, composure, and graciousness: a serene old man; when applied to nature there is a suggestion of mellowness: the serene landscapes of autumn. Tranquil implies a command of emotions, often because of strong faith, which keeps one unagitated even in the midst of excitement or danger.

Did you mean unmerciful?