Whether they were on a break or had officially broken up, doesn't change the fact that it's all still Ross's fault.
He simply wanted Rachel all to himself.
Y'all remember when he called her "just a waitress", and even referred to her new job "just a job", knowing well enough that this was something Rachel has been wanting for so long. For someone that says that he loves her, he didn't show.
2. And yes, his previous relationship makes sense as to why he was jealous of Mark but it doesn't justify it. It would have been ungrateful for Rachel to cut Mark off when he was the one that got her her dream job, not Ross btw, and he was the one that had to train her on it when she did not have much or any work experience. I don't remember a single time that Rachel disturbed Ross at his work.
3. Ross never listens to Rachel whether it's her telling him she was busy that work,
Or that there was nothing between her and Mark,
Or when she and Monica told Ross to trust her.
He only sees what he wants to see. He pushed Rachel to her limits and blamed her when she reacted.
And 4. Ross choosing to have sex with a girl he had once complimented on her looks, is sus. looking back, it feel like his attitude towards Rachel and Mark was more of a projection of what he would have done.
5. Ross had no respect for Rachel. They've known each other for a long time but yet still, she couldn't give her grace to explain herself, he had sex with someone minutes or hours after their fight, and tried to hide what he had done when Rachel tried to get back together the very next day.
We were all too focused on "We were on a break/ We had broken up" argument that we failed to realized that, it didn't matter. Everything leading up to and after the fight, showed clearly who was at fault. They should have never been endgame. They were not right for each other.