They could not have started off more amiably and on track, and they had a tiny disagreement over whether US or EU helps more but that's not at all the schism that started when Trump harped on at least 3 times about how both Russian and Ukrainian parents were losing children and Zelensky had to butt in to say the Russians deserved it, then followed up to drill that point again instead of being conciliatory. He got on track by inflating Trump's ego at the end but it was the first warning shot. Both they remained pretty aligned even through the first few pointed journalist questions but once the meat of security deals got discussed, Zelensky got into a tirade about how Putin is evil and Ukrainians are glorious and Putin must pay all the costs and they will compromise on nothing. Trump goes on for 10 minutes in various questions about he's trying to stay in the middle and make a deal. "I'm not aligned with Putin I'm not aligned with anybody I'm aligned with the united states of america" and he finally calls out Zelensky for his hatred of Putin. "And I can tell you the other side isn't exactly in love with him either". It was clear they weren't in agreement but they were still grudgingly aligned.
I think up until JD Vance jumped into the blood moor to ambush with an offscreen telestomp zerk, it was salvageable for Zelensky. Vance didn't immediately go for the throat with his first remark but Zelensky walked right into Vance's overture for diplomacy by fully splitting open the wound from above. And then it was Vance who finally gave up the neutral and amiable conference and just started slamming Zelensky around the ring and jumping off the ropes. Zelensky had made it clear he wasn't going to sign onto actual diplomacy by that point, but would have kept up the niceties. And like Xisten says, Zelensky made it so much worse by arguing the point and saying the US would be the ones to suffer. Trump listens to Zelensky's fatalist ultranationalist spiel and probably thinks, "I don't like him, but I can work with him". He starts saying Americans will start dying and Trump finally wakes up and joins Vance's side. Trump forgot everything that was said up until that moment and had a guy in his face saying "No you!" and all the good will went out the window. That's when it went crazy and we saw the clips.
At the end of it all, whether Zelensky is a puppet or dancing with his own strings cut or seeking the EU masters, he represented a full partisan Ukrainian ultranationalist worldview. No compromise, no acknowledgment of his side's wrongs, Russia is to blame for everything, Ukraine is perfect, all the DPR / LPR / Crimea all belong to the west. Russians aren't humans and their lives don't matter, Russians commit war crimes and Ukrainians don't. He wouldn't budge even an inch, not even with his back against a wall here. Well, what happens to people who won't bend? And Trump didn't give one fucking shit. Trump put his cards on the table and said he doesn't care, that he represents American interests not Russian or Ukrainian, its not our fight.
But boy whether Vance brought the necessary fire to set alight whatever follows, damn did he come off as being a real asshole.