I can understand the actions of myself and others, at least to some extent. Or, really, I can tell myself I understand it. It may be due to personal history, culture, genetics, physiology, circumstances, and so on.
At the same time, I can take responsibility for my own actions. I did it. I made the choice. I lived it out. Whether it seemed wise or not, whether it helped or hurt others and myself. (At least as far as I can tell.)
For me, these are two sides of the same coin. I can understand my actions, to some extent. I may see it comes from unhealed wounds and trauma, or from more clarity and kindness. I may see it comes from how I relate to this in myself. I may see that any of my actions have infinite causes. And I can still take responsibility for them. All of these are pieces of the puzzle.