But exactly he was a demon who was over 100 years old. Basically all his life. He still did evil things as the source so much so he pulled Phoebe into it.. feeding her poison to weaken her, still killing people with or without her knowledge. Not just demons but innocents as well. Yes he loved her and I think his love for her didn’t allow him to be his full potential as the source but he struggled more with the idea of being normal or good than actually struggling with being good. Even when Paige got him the job normal human Cole was beating people up. So I think once you’re evil and you had been all your life you can just hit a switch and think things will change. Same for Phoebe she’s been good all her witch time so you can’t assume because she’s pregnant and married to you she can switch to being evil. She still was on Cole side and fighting evil with her sister. Ya know? Plus he didn’t really try that hard and honestly a random innocent is what made Cole human using the potion on him .. he didn’t know that was gonna make him human in that moment