Are you sure he didn't deserve to die? If a man broke into your home and threatened your livelihood what would you do? I get what you're saying but Mel signed his life away when he smashed that glassThat detective was annoying but he wasn't someone who deserved to die. If his worst qualities are being a coke head and self righteous then he might as well be a saint compared to 90% of the people in Ozark. Jonah was the last family member who didn't agree with how Byrdes were moving and was ready to move out. It was pretty much out of character what he did in the end compared to what he would have done in the past. That whole accident scene and reasoning being so was just lazy writing
I think putting the entire blame on the car accident as a catalyst is a mistake. The fact that the entire family was reunited in the van beforehand was establishing what people blame the accident on. The kids forgave Wendy for finally admitting to her bullshyt and telling the whole truth. The accident didn't cause a bond, it solidified one
And ultimately you gotta look at the context of the Byrdes during that final scene. Jonah killed Mel because he was threatening their newfound freedom that they've worked 4 seasons to attain. The entire family was on the same page of "Nikka we worked TOO hard for you to pop up with THIS bullshyt"