He also had to do it quick, otherwise she'd sense his presence and his hostile intent. Stand users, especially bound by blood, are tied by fate (a major theme in JoJo). So he couldn't just risk deception. That intent to kill would have been in his soul so he'd be caught out by Trish instantly. Just like in Stardust Crusaders with Jotaro, Joesph and Dio, sensing each other both physically and their emotional state.
That's why the boss had to be there. He couldn't risk her having cold feet and getting away. King Crimson's range is pretty small, so she could have easily escaped him. He couldn't predict or really care about Buccarati's reaction. Even so, he backs the fuck off once the crew comes at him. And he doesn't show himself for a long ass time. He only does when they're inches close to figuring him out and he has no choice and is super fucking pissed about it.
I forget how he even found her, I don't think the boss knew of her existence until it was way too late. He used Giorno and co. because they were the closest to her and he had just been made capo. And the boss was NOT going to come out of hiding at all. He even hated coming out as Doppio, he only did it to kill Abbachio. He resorted to basically decimating the entire city of Rome by using two sociopaths who were so disgusting it lead to a seven page MUDA, instead of coming out and doing it himself.
He ONLY comes out when the stakes are at their highest. He doesn't risk shit. That's why he didn't kill Trish earlier. He didn't know, and wouldn't risk it until she had no possible chance of escape, which is why he hides on top of the elevator. Even if he couldn't predict Buccarati's betrayal, as I said before, it didn't matter. King Crimson is just too powerful for him alone to take on. But he books it when he's outnumbered.