What happens on a chemical level to make Methane so bad?

Methane's greatest impact is more about global warming than ozone depletion (as hydrofluoro-, hydrofluorochloro-, and hydrochloro-carbons do the real destructive work in that area). However, methane will react with OH radicals -- products of ozone interactions with the chemicals above. Methane is 20 to 30 times more effective greenhouse gas than CO2 (carbon dioxide), so the man-made increase of that gas to the atmosphere is a major concern.