I did something like this for Sam, now it's Robby's turn.
I'm gonna leave a space so everyone has time to boo the title....
Ok, is everyone ready for this?
Let's take into account Robby's home situation in Season 1. He has clearly grown up without parents around. Johnny, as good as he is, left Robby and Shannon for years. Robby's mother was no good influence, as she spent more time finding men and casting them aside. Robby clearly spends time alone when he is at home.
Robby joins Trey and Cruz, and yes, I am aware that this is not a good thing, because they are clearly idiots. However, this occurred due to Robby lacking a mentor or parent. Robby is clearly the lackey and following orders.
Let's look at Robby after he starts training with Daniel. Robby clearly shows a conscience. That doesn't come out of nowhere. Robby always had good in his heart, he clearly loves his mother even when he is a troublemaker. Daniel doesn't magically turn him good, he helps Robby get confidence to move past his issues.
Robby says "It's ok, dad" after the tournament, yet you all are hating him still. I highly doubt that one moment was going to fix years of issues. Not to mention Miguel fought with a few underhanded tactics, and since Miguel learned from Johnny, Robby reasonably thinks that Johnny taught Miguel to use these tactics, even if he knew his own son was on the mat.
Robby stays out of scumbag territory for the most part during Season 2. He tries to fight Trey and Cruz for the views, sure, but keep in mind he clearly wanted to return the stuff they stole. That is surely a sign of goodness. Also, he does hide the medal of honor, from Sam, but he confesses. Yeah well guess what? EVERYONE has lied at some point in their lives. I'm talking about everyone on this wiki and in the real world, not just characters. To drag Robby through this is self righteous, especially considering Robby told the truth later, which is certainly something many of us in the real world could work on.
Note that during the fight, Robby was sad when Tory revealed the Miguel-Sam kiss, yet he still tried to defend Sam, and wasn't trying to hurt Tory. Deny this all you want, but Kreese is right when he says Robby only tried to stop the fight. Miguel is my son, but he escalated things when he saw Robby. Now of course one could argue that Miguel saw what he saw (Robby putting his hands on Tory and kicking a Cobra Kai, Miguel's friend).
The key here is to think from Robby's POV. Robby did not see Miguel coming at all and his intentions were to defuse the situation. All he knew was that he was defending himself from Miguel. Plus, he is correct to an extent when he says Miguel took advantage of Sam, after all the only thing he knows is Miguel kissed a drunk girl.
Now comes the big part: the KICK OF DEATH! Ok, Miguel didn't die, but y'all know what I mean.
This one, I can't exactly absolve Robby of, because Miguel said sorry and Robby still beat him. Robby however clearly showed remorse after the kick. It might be a paltry defense, but clearly he was unaware that Miguel would go over the railing.
Ok, onto Season 3. Robby is clearly aware that he is a fugitive here but he is NOT PROUD OF IT. Notice how he clearly shows remorse when talking to Daniel and is clearly apologetic.
When the police show up, Robby is taken aback, seeing this as Daniel going behind his back. I am not saying Robby did not deserve at least some karma, but it would have been preferable for Daniel to warn Robby of what happened. Robby obviously shuts Daniel out, not wanting him to visit and refusing to answer his calls. This is a fairly normal response to someone who has turned in a juvenile who has shown remorse for their actions and thought the parent/guardian would always be on their side.
It would be quite ridiculous of anyone to suggest the Robby-Shawn feud was necessary to teach Robby a lesson. Robby was clearly trying to stay out of trouble, and stood up not to start a fight, but out of concern for a victim of bullying. His only priority was to finish his book. Even when Shawn slut-shames Samantha when she comes on TV (sorry for the language I did not know how else to put it), Robby tries to restrain himself and sticks with the Miyagi-Do defense way, clearly showing some respect for Daniel despite the whole turning him in thing.
Robby fights and gets put down, badly. Now onto the Johnny visiting Robby situation. Johnny is stuck visiting Miguel in the hospital due to Yaya Diaz's instance. You can hate Robby and defend Johnny all you want, but Robby DID NOT KNOW about that whole thing. Why is everyone getting mad at Robby about something he literally had no knowledge or control of? Robby was told Johnny would visit, and Johnny didn't show up. That is the only thing Robby knows.
Now, it must be noted that Robby is clearly trying to hold some faith in Sam, despite the kiss with Miguel. He not only defends her against Tory, he is trying to respond to Sam's email, and is only unable to do so because Shawn was being an idiot.
Also, when he catches Miguel with Sam, he sort of has a right to be concerned. Miguel and Sam held that position for a suspiciously long time, and they have a history, so why wouldn't Robby be annoyed? People defended Miguel when he drunk punched Sam, but come on, Robby is sober and still sees something is off.
Now he did not apologize to Miguel, and yes he tried to throw a punch at Miguel, but as hard as it is, remember Kreese is in his head. Also Miguel and Sam did kiss, so Robby's suspicions are still understandable.
Here's where Kreese comes in. Kreese gives Robby advice that is unfortunately pretty valid. This is the real world. Defense only won't always work. You cannot get rid of your problems unless you take them on head on and shut them down. Plus he also mentions the fact that Robby did not start the fight at the school, he tried to end it, and he is the ONLY PERSON thus far to acknowledge this, so it's pretty clear why he would be intriguing to Robby. Also, Kreese plays with Robby's mentality and plays the "you don't deserve this" card. Again, Kreese is the only person thus far who has acknowledge and understood what Robby is thinking, at least in Robby's mind. Kreese ends with introducing Robby to the Cobra Kai mantra.
And guess what? Robby strikes first against Shawn and his gang AND IT WORKS. Sherlock doesn't need to be called for this. Kreese has understood and acknowledge Robby's pain and thoughts, given him valid advice, and helped Robby solve his problem. In other words, Kreese has won Robby's trust.
Don't go saying Johnny would have done the same. Even with that mess in the cafeteria, it was uncool of Johnny to bring up the hospital point, also Robby has more than once seen Johnny shove him aside in favor of Miguel, and this is the breaking point.
Now, Robby does not become an instant jerk when he joins Kreese. He is grateful to Kreese for the advice and for GIVING HIM A HOME. Very rarely does someone who is being manipulated become aware of it.
Now obviously Robby tries to leave because he doesn't want to rub shoulders with his enemies. That's fairly reasonable. But Tory gets in his head, and Robby stays. He's still not a complete jerk because he calls out Hawk drinking. Of course he does steal the snake, but he does it because he wants to win his peer's trust and not have to spend every day looking over his back. Not the best excuse, but it is somewhat valid. Kreese praises Robby in front of the other Cobras. Kreese is corrupting Robby even further.
Now for the season finale. Robby's corruption is shown in its fullest extent. Before fighting Johnny, he openly admits the years of self blame he felt, but then brings up something Kreese told him. Robby's pain is shown and now it has been combined with Kreese's teachings.
Robby does attack Johnny, but this is YEAR'S WORTH OF ANGER combined with the teachings of a sadistic boi (maybe I'll defend Kreese later, idk).
At the end, Robby stands with Kreese because he is pretty much the only one Robby has no beef with. Kreese is the only one who has not hurt Robby in any way.
So, think big. Put yourself in Robby's shoes. Try to use a GOT mentality and make each character a POV character, not just use a good vs bad mentality. Empathize with his pain. Don't overlook his failings, but don't act as if he is the worst person in the world (A LOT OF PEOPLE DO THAT).
TL:DR: Have a heart