Why Harlan DeGroat had to do that to Rodney Baze Jr.?

He killed Rodney because Rodney was a witness to Petty's murder. DeGroat's real issue was with John. John still owed DeGroat more money. The fixed fight didn't settle the whole debt. Before they leave, John asks him if everything is good between them. DeGroat says, "well yeah, as long as you brought me the rest of my money." Petty says nothing. So DeGroat asks, "don't tell me you came all the way down here without the rest of my money." Petty insists that the fight was supposed to settle the whole debt and that was it. At that point, DeGroat realizes he's not going to get the rest of the money so he decides he's just gonna kill Petty now. When he kills Petty and he and his crew drag Rodney in the woods. He has a look that says "sucks I have to do this kid." Then tells Rodney that he should've kept better company. Then he feels a bit bad about it so he tells Rodney not to look at him. As if seeing Rodney's eyes as he kills him will haunt him because even though it was a messed up situation he did like Rodney. Thought he was a tough kid. But he witnessed a murder.

It's because DeGroat was tired of Petty. In the beginning of the film he tells Petty that he should just put some lead in it. After Rodney blows the dive the deal was off, so he kills Petty and Rodney just happened to be there. That's why he tells him he should have kept better company.

I was wondering this myself when I watched it last night. I sat wondering, "What the crap, why?" It bothered me so much I Google'd it and found this. I honestly can't come to a real conclusion other than Harlan DeGroat (Woody Harrelson) needs to be depicted as miscreant with limited tools to navigate life. Violence is his means drugs his entertainment. If it's not clear, DeGroat is a crime boss not just a low life leader of a rural area. Killing Rodney and Harlen might simply be an example of his power over the area, he can openly kill anyone he wants and the law can do little to stop or reprimand him. Maybe the death of the two would further his image as a crime boss.

On another note I would find it understandable that he might kill someone indebted to him to send a message to others he might deal with vs collecting money he most likely will never get. So Petty's (Dafoe) death might serve that purpose and killing Rodney, I might assert DeGroat might not need a reason or that Rodney exemplified the same contempt for life and violence DeGroat had, so he offs him. When DeGroat first interacts with Rodney there's serious tension, DeGroat even goes as far as slapping Rodney, behavior reminds me of Alpha dog pack nature. Maybe DeGoat sees Rodney's lack of fear as a danger seeing there's always some weird dude following him around almost the whole movie, everyone is subject to him hence the initial drive in theater fight.

Another explanation... It could be one dimensional thinking of how to progress the story. For a revenge story it is slow and even timid.

I had the same question but then I thought: "Does it really matter why Woody killed Rodney?" It is a senseless killing and that's the point. In the first scene of the movie Woody is established as someone easily resolving to extreme violence if something rubs hm the wrong way - and this whole thing with Dafoe and Rodney clearly bothered him. Russell on the other hand is struggling with killing, because of the accident where he was responsible for killing someone innocent. He hesitates and passes on shooting the deer, but later he shoots Woody with great resolution. I thought the last scene was beautiful showing him pondering it all. I really liked the movie ...

Harlan killed Petty because he owed him 25k and wouldn't pay up. Rodney was just in the wrong place at the wrong time. Leave no witnesses. Before killing him Harlan even says

You shoulda kept better company.

It's that simple and made perfect sense to me.

Harlan kills Rodney because he witnessed Petty being killed. However the fight was supposed to clear up his debt with Harlan as John thought. But afterwards Harlan changed the deal on him and Harlan still wanted his money. So it was principle of you owe me. You can pay me my money and the dive was to cover his inconvenience or you don't and I kill you and I still get my money from the dive. Drug dealers aren't always moral people. However they are big on trust and loyalty. Once you break that trust you don't get a second chance. Such as John Petty didn't.