I concur with your anger and frustration. Teppei was so strong and admirable throughout most of the series, but then he ended up disappointing me so much. If not for the hope of still being able to realize his dreams, he should have stayed alive for his wife and son at the very least. His son now has to grow up without a father...Teppei of all people should know how that feels on some level. His wife, who was willing to stick with him through thick and thin, is left a widow. And the memory of her father who treated Teppei like his own son should have weighed heavily on Teppei's conscience. In one of the last scenes when Teppei's final thoughts were revealed, it really seemed like he didn't have any other options and that he was no longer in control of his own destiny. "Why won't I be able to see the sun tomorrow?" You would have been able to if you didn't put a bullet through your head, dammit!!!
I actually plugged my ears during what felt like an infinite pause when he held the rifle to his chin because I wanted to be in denial for awhile. I suppose this is the ultimate punishment for his father, but still I feel the price is too steep. Teppei was a man so hungry for his father's love that he starved.
A few comments about some of the other characters. What's up with Ginpei? He admired Teppei and wanted him to succeed so the family could be "saved", but he never told Teppei anything even though he was aware of and disgusted with what Daisuke was planning to do. In the end, he admitted that the ones who indirectly caused Teppei's death were him and their father.
Manpyo Daisuke. Realistically, I doubt a man with his pride and personality would have had three more children with the woman he strongly suspected slept with his own father.
Aiko. The writers tried to humanize her here and there, but she was too far gone for me. The best thing she did was help prevent incest.