This. He basically negated the agency of everybody who loves him and threw his life away.
Naturally this is terrible for the Gems sworn to his mother. They love him and him dying like this means Rose would have died for nothing.
And Connie had basically lost the love of her life, after being rejected by him in the end. Which was what he had sworn not to do, even asked her to never do. They would always fight together.
But he had to go it alone; no desperate last stand, no throwing everything at them and falling short.
He just ignored the wishes of his family and took himself away from them.
And then he just comes back like it's all cool. Like he didn't just yesterday accept certain death and slap his proud warriors in the face.
Like suicide isn't a selfish act.
Of course Connie is being upset about her feelings when he gambled his life and that is not exactly grateful. But he also denied her the chance to fight, refused the fusion that would see them stand a chance. He decided that she would live and he would die when she was more than ready to follow him into the jaws of hell. And that is a breach of trust that would be easier forgiven had he actually died and ended the conflict. But nothing of the sort happened. He just pissed off everyone and got the Diamonds angrier.
Sure, this is all not precisely rational. But emotions seldomly are. And emotions are what the show is about.