Neighbors in West Brookfield gathered in the small central Massachusetts town’s center yesterday to comfort each other in the aftermath of unspeakable tragedy — the unsolved killings of a mother and three children that has left townspeople both devastated and fearful.Slain family members Sara Bermudez, 38, and her children, Madison Bermudez, 8, James Bermudez, 6, and Michael Bermudez, 2, were honored yesterday at a vigil at the town’s First Congregational Church.