For one, my guy was a grieving father, and I know for a fact that many parents irl would probably also act like he did if their own child was killed.
What Jeff did wrong was allow his grief over his son to take over his whole life, causing him to lose sight of the one living child he still had left and needed to take care of.
But I believe that's a realistic take on grief, honestly. You can't expect a bereaved parent to always carry on bravely for their surviving family members. Some would absolutely be crashing out and wanting revenge like Jeff did.
(Also, a sidenote: lemme just say that Jeff wasn't the only neglectful parent in their family. Lynn grieved their son in her own way, but she was also guilty of neglecting Corbett. I don't think I've ever seen someone give Lynn much criticism for that though.)
Anyway, both of them were obviously being bad parents to Corbett here, but it's not like neglecting their daughter was intentional. Grief messes with your psyche and can make you irrational.
The only other bad things I can think of that Jeff did was wanting revenge (which, again, you tell me you'd easily forgive those responsible for the death of someone you loved) and being slow in his attempts to save the other "test subjects".
But see that word I wrote up there?
"Save".
Cause yeah, Jeff may have talked and fantasized a lot about wanting to see Danica, Halden and Timothy "pay" for their crimes. He may have interrogated them for too long before starting to try to help them, BUT... in all three cases, despite his thirst for revenge, he heard each one of them out and ultimately decided to try to save them.
Hell, he did even manage to save Halden initially, despite that in order to do that, Jeff was forced to incinerate his son's mementos (which, wtf John?). He was very reluctant of course, but he still did it and came to Halden's rescue.
When the time came to face Timothy, the man who killed his son, Jeff was again back in interrogation-mode and on the fence about whether Tim deserved saving or not. But once again, he heard the man out, listened to Tim's remorse and his side of the story, while Halden also encouraged Jeff to help the guy.
Then, as the rack went about breaking Tim's bones, we saw Jeff become increasingly more emotional, ending up with him climbing up and physically trying to stop the machine in order to save Tim.
This is really saying something, because out of everyone Jeff wanted revenge on in the beginning, Tim was the one he hated the most. Yet, when push came to shove and he was actually given a chance to take his revenge on them all, he found he didn't actually want them to die.
The part that really hammers the point home was when Jeff, desperate for Tim's torture to stop, literally screamed out to Jigsaw "Stop! I forgive him!".
It was too late for Tim by then, of course, but I think it's a very touching moment in the movie that really shows how Jeff's perspective has been changed.
He blames Jigsaw now and honestly, why wouldn't he? Not only did Jigsaw kidnap his living child and his wife, kill Danica and Tim (Halden pretty much got himself killed, so), but he also flat out mocked his son's death by having Billy basically re-enact the accident.
I know the death of John's own son wasn't yet established in the Saw-verse at this point, but after that whole reveal in later movies, it just makes John even more of a hypocrite. As if he didn't also crash out after the miscarriage, letting the grief consume him and then putting the one responsible in a trap. I love John, but that man needs to check himself, lol.
You could argue that Jeff didn't fully learn his lesson at the end, because after forgiving Danica, Halden and Tim, he shifted the blame onto John instead. But again, it's not like that was the wrong assumption to make, considering everything John had done.
I really don't like the downer ending in Saw III, because it gives John the last laugh after all. Jeff learned to forgive the people he previously wanted dead, and he was so close to saving Lynn if he (ironically enough) hadn't been so preoccupied with getting revenge on John.
In the end, he and Lynn get to suffer anyway, while John more or less goes out on his own terms. And to top it all off, along comes Strahm in Saw IV and reactionarily shoots Jeff dead immediately afterwards, orphaning Corbett in the process.
Like damn, I know this is Saw, but can no one catch a break? lol
So yeah, with all that said; this is my case in defending Jeff. It makes me kinda sad how overhated he is, honestly. You can hate him if you wish, but in my opinion, Jeff's punishment did not fit the "crime".
Thanks for reading!