I believe that it is a combination of the two. He definitely had some odds stacked against him that determined the quality of his life as he lived it. However, everyone is given free agency, and the ability to make choices to either better or worsen their lives. Ethan's fatal flaw was his feeling of obligation and duty to his family members--that is what kept him from making the choice to leave them behind, and live his life in his own way, forging his own happiness.
Circumstances gave him sickly parents, which brought him home from his studies. He was a victim of that--what only son wouldn't come home to care for his parents? That did determine his life for a while. And, it even brough him Zenobia, and marriage in that way. However, after his marriage, he was not a "victim" to the farm and its duties. He could have sold the farm, and moved somewhere else that would have brought him more success. Zenobia didn't get sick immediately, and if he had been more proactive in pursuing his dreams, she could have gone with him back to school, and he could have sold the property. After Zeena gets sick, he does become more of a victim to the circumstances--what good husband wouldn't do what they could to help their wife? However, he could have still at least sold the property and moved somewhere that would have afforded them a happier situation.
When Maddie enters the scene, he makes the choice, in small ways every day, to let her into his heart. He was not a victim of circumstance there; he knowingly and willingly chose his feelings for her, or at least chose to let the feelings remain and grow. He chose to be with her, to let her know his feelings, and to make plans with her. And in the end, he chose to take that fateful trip down the hill. All of these things were within the realm of his control, and could have been avoided. The consequences could prompt one to say "Poor Ethan!" and declare him a victim, but the thing is, he himself made the choices that led to his situation. So, in some instances, he was a victim of circumstances, but in others, he chose his fate. I hope that helped; good luck!
Ethan Frome had always been a victim of circumstances with the exception of the short period he had spent away at school. His mother
His illness required him to quit school and move back home to care for his mother. Zeena is brought in to care or his mother and is the only female he has really been around. Zeena took care of his mother. When the time came to send her away she as quite panicked by the idea of having no where to live nor a way to support her self.
Ethan is a fair man who was lonely and had worried about bills and hurting Zeena. He marries her and then meets Mattie whom he falls in love with. If it had only been different time and circumstance, Ethan could have had a happy life with Mattie, but he is married to a bitter, boring, whiney, and hypochondriac woman instead.