Antiseptic/disinfectant methodology involves a biocide mechanism that is either static (inhibitory) or cidal (killing) for which I think you are asking.  For microorganisms to live they require functioning cellular components such as proteins, nucleic acids (DNA, RNA), lipids, carbohydrates, metabolites, etc.  Many of these organic molecules possess heterocyclic ring structures/components such as  certain amino acids in a protein or nucleic acids in DNA/RNA.  These chemical structures or functional groups have to be intact for the rest of the molecule to be functional.  UV irradiation of these chemical structures involves free radical chemistry in which certain chemical bonds, double bonds in a ring structure for example, become broken due to the light energy. This in turn exposes/produces highly reactive electrons that can cause other bonds to break/form between other molecules leading to an unfavorable/unnatural/abnormal  covalent chemical bond and so the molecule is no longer functional and the organism dies.  Certain molecules are not meant to be combined with different molecules to be functional.  Sometimes unnatural polymerization of the same or different molecules will occur, again leading to non-functional molecules that can't perform the life functions in the cell.  You might want to get obtain a brief overview of photo-chemistry involving UV radiation because I tried to provide a very general answer.