IMHO the toxicity actually mostly comes from Wrathion's side. I love him, he's one of my favourite characters, but he is a scumbag and the punch not only was well deserved, my man needs to be a put on a leash. So judging by your post you're not very knowledgeable on the Pandaria-Draenor-Legion part of the timeline, therefore let me tell you a few things you missed :D
1. Wrathion was afraid of Legion's returning, and had some really wild ideas about how he wanted to prepare Azeroth for Sargeras' coming. For once, he wanted for one of the factions to finally win and absorb the other into itself, so that Azeroth meets the Legion unified to prevent in-group bickering which cost everyone their freedom, their planet and their lives. He chose Alliance and through Varian would put an end to the war and finally destroy the Horde.
2. Not only that, but he was also acting totally on his own. This self absorbed fool was surrounded by people who have lived decades, hundreds, thousands of years more than him and understood the world of Azeroth intricately, yet he never cooperated with anyone and basically decided the future of Azeroth on his own.
3. When Garrosh was finally captured, he hoped for Varian to kill him and every other Horde racial leader present at the meeting when Vol'jin was appointed Warchief. He was really disappointed when Varry did not, and decided that the best course of action WAS TO FREE GARROSH 💀
5. Garrosh escaped to an alternate timeline Draenor with the help of a bronze dragon. WoD happened. I personally blame Wrathion for that expansion. He brought THAT upon us lol
6. Alternate Draenor had alternate Gul'dan. Gul'dan, a powerful and scary dude who was thankfully killed during the first war, whose skull Illidan ate to become what he is today, IS NOW BACK. Of course, alternate Gul'dan contacts the legion, becomes a warlock, etc etc etc
7. WoD is over (thank fuck!!). Gul'dan brings the Legion to Azeroth. Wrathion caused that. Varian is killed. Wrathion also caused that. Anduin the poor guy is going through the roughest time of his life as to date (Shadowlands are right around the corner though lmao) so there's nothing toxic about him ignoring letters from a dude who knocked him out while Anduin was trying to stop him from freeing the scariest war criminal Azeroth has ever seen, and then very much directly caused the invasion of the Burning Legion, unimaginable suffering and countless deaths, including his only surviving parent. (His mother was killed during communist riots in Stormwind)
8. In general, Wrathion is a super clueless person due his egotism and how little he thinks of others. Trying to contact Anduin after everything he did was outrageously tone deaf, and not in a "pure uwu babygirl" kinda way, but more in a "I'm uniquely competent and the fact that you hold a grudge against me for my masterplan going south is your own personal failure... anyway, here's more about how I spent my day" kinda way.
9. Also, Anduin was being influenced by N'Zoth in that cutscene. Andy is the real babygirl here, bro deserved so much better than what he got 😭🙏
10. Also, this might be a personal gripe, but Wrathion is kinda ruthless. Just to compare him to Sabellian (who I'm not a fan of, frankly, but this is a good contrast) - while Sabellian was trying to fight the corruption and actively working on purifying fellow black dragons from Old Gods' corruption, Wrathion did the exact opposite. Back in Cataclysm when we first met Wrathy, he hatched in the basement of a bandits' headquarters and hung around with those rogues for a long while. One of his closest advisors who was very intensely loyal to him, and stayed lotal even when Deathwing returned, was Fahrad. Fahrad kept his black dragon identity secret and helped Wrathion cull other black dragons, since Wrathion judged them beyond salvation (not even having tried or even considered any other way btw). In the end, Wrathion did sus out Farhad as a black dragon and ordered for the player to kill him, while Fahrad pleaded with Wrathion to spare him because he was in control of the voices in his head, and it was Wrathion's words that triggered him to go off the rails and attack us. Which, he could've done at any point. I am sure that he had plenty of moments to surprise attack Wrathion, and I frankly doubt a whelp could defend itself from a fully grown, rather old drake. Additionally, to have someone be so loyal to you, to spend so much time with them and not even grow attach enough to try to help them instead of killing them due to an affliction they did their best to control, which they were not to blame for? I think that's really messed up. And Wrathion is definitely not a positive or trustworthy character. He is the toxic one here lmao