From what i remember Obito himself stated later that it was not for rin or kakashi but bc madara manipulate him like how this World is only pain and hatered
Wake up to reality nothing goes as planned in this accured world blah blah blah everything is merely pain, suffering and futility blah blah blah and so on
it doesn't make much sense for Zetsu, Konan and Pain to be surprised to see Obi's Kamui. Even if Itachi had never seen Obi use Kamui (which I'm pretty sure he has), he knows that "Madara" would have some weird jutsus, even more so that each Uchiha awakens a different Mangekyou Sharingan ability, the Zetsu I don't even need to explain right, he's lived with Obito since Obi was a teenager and I think Pain and Konan have already seen Obi use this ability, since Konan was able to prepare a battle against Obi and she considered the fact that he dematerializes. And the shark that I forgot the name of, thought that Obi was Madara, so it was to be expected that he would have abilitys. and I watched Naruto in 2013-2019 so maybe I missed something, but either way, it doesn't take away from the fact that the reaction is...
He is just angry on shinobi world the counterfeit world of theirs for letting people die especially Rin he is not able to recover from the pain the loss
