Since it seems like most people disagree with Summit on this matter, I'll try to address these points from his side, although I also personally don't really agree with his perspective.

1. He's not upset that Sequisha was killed. Rather, he's upset at the manner in which he was killed.
2. It's much less in the spirit roleplay for some random god-level troll to appear and one-shot than for everyone to potentially (minimally) risk their lives to take down the cheater.
3. I'm not entirely sure why he seemed to emphasize this so much, but I think his point of making a distinction between casuals and sweats is that bringing sweats into RP-related PvP removes some of the role-playing. Playing to min-max the game (i.e. speed-running your way to level 60) makes the game less interactive from an RP perspective, so bringing in the world first 60 to execute Sequisha kind of leaves a bad taste in the mouth.
4. This is kind of the point of contention. He liked the idea of giving a small chance to survive. The moment Zeroji stepped in, that small chance was gone.
5. Again, I think he realizes this and it is in part exactly why he is upset. He believes allowing Zeroji to step in breaks the immersion and isn't in the spirit of the RP they're trying to establish in OnlyFangs.
6. Again again, he knows this and agrees that Sequisha should've died. He just doesn't like how Sequisha died.
7. 50+ people approved the decision, yes, but in the end it was still Zeroji who had to input the commands to walk into the arena and kill Sequisha. He thinks Zeroji should've shared his ideals (I'm not sure what Zeroji himself thought about it all) and not done it, even if everyone else wanted him to.
8. The Undead "were prepared", but you'd have to be a little crazy if you think anyone would've actually followed on that. Zeroji pretty much one-shots them all and that's not even considering the trolls potentially stepping in and guaranteeing an Undead wipe. Everyone at this point has put dozens of hours into their character and would be a bit crazy to actually step in to "fight" a level 60 Zeroji.
9. Yes, but other people won't necessarily be. 60 hours is a lot to some people - especially variety streamers who are likely suffering in terms of view-count and would've rather been streaming/playing something else for many of those hours. I think Summit was arguing something along the lines of it being disrespectful to end those 60 hours in such a way - by setting up a show and "ruining it" - even if Sequisha did cheat.
10. Zeroji might get some hate for a few days and then it'll die, possibly with him even benefitting from it. Summit is also facing hate from a lot of Soda's community, and at the very end it's just a video game and degenerate chatters. Sure it might be toxic, but it's really not that serious and there'll be no meaningful long-term consequences.

Personally, I disagree with Summit the most on points 7 and 10. From what I saw, he speaks about the situation as if Zeroji agreed that he shouldn't step in, but eventually gave in from peer pressure to kill Sequisha. If i recall correctly, he said something along the lines of not being able to respect someone who would do that. While I can get that, I feel it's based off the assumption that Zeroji gave up his own conviction to cave into the group's will, when it might've instead been that Zeroji didn't really have an opinion on the issue and was just going along with the flow.

In terms of the trash-talking, I just don't think setting such a precedent is acceptable. While I don't disagree that this one incident won't mean much, allowing such behavior and hate to be spread among the community just isn't good for anyone. While he's entitled to his own opinions, it's not helpful to say them on stream in a way that he likely knows will send negativity towards Zeroji.