To get straight to your question, for some "blame" is a difficult question, with many different answers, and it's probably more than one of those answers in truth. For me, the answer is easy. I apportion 1% of the blame to my genetics. Men on both sides of my family have carried weight around their middle, some have died from conditions related to that, but I'm the first (known) diabetic on either side of my family.
99% of the blame goes to yours truly - me. I saw the consequences of unchecked weight gain in those relatives mentioned, so although the genetics are out of my control, I should have made better decisions and choices in life to avoid getting to 25st 11lbs at my peak weight. I should have eaten better. I should have exercised more. More than anything, I should have stopped making all the excuses in the book as to why I shouldn't/couldn't change things. This goes double for when I met my wonderful wife and applies a billion percent more having had two amazing sons in the last 10 years. I've let them down by letting myself down, I've caused them all so much worry and stress with this, and for that, I'll never, ever forgive myself.
Some might not like seeing how harsh I've been in self-reflection, however for me personally, it's important to own my mistakes and errors in judgement. It's how we learn and it's how I ensure I don't repeat those mistakes. I've made peace with those past mistakes and errors in judgement, but I can't forgive myself for them. I don't seek reassurance or consoling here with this, that lack of self-forgiveness drives and motivates me. I'm 3 months in now since diagnosis. I've lost a load of weight, I've got a low carb diet that I love, my BG readings are in normal range, I'm fitter and healthier every day, and I can do more than ever with my kids. Beyond that though, I'm happier than I've been in 20 years when it comes to how I feel in myself. It took a T2 diabetes diagnosis to achieve this, without it I don't think I'd have ever stopped with the ridiculous and pitiful excuses, but so far I'm on the right track. I have my second hba1c blood test next week, following my initial diagnosis one of 83. I'm hoping I'll see a significant improvement to match the complete overhaul of my life that's made me so much more content.
All that said, I aspire and hope to one day be able to have this wise an outlook on the matter of blame...