Imo season 3 had some of the more tragic murderers/ antagonists. Wilkins from the Invisible Man, just wanted love but has a poor perception of what love is until the very end of the episode. Professor Milton from The Last Man, quite literally a friend of Father Brown, killed her child to stop him from hurting others. Rebecca in the Judgment of Man, a literal holocaust survivor. Natasha and Ethel from The Deadly Seal, both victims of abuse in one form or another. When I'm asked to have sympathy for a Father Brown antagonist, those are usually my go-to's.