He'd staked out the area meticulously, eminently pleased that the church entrance didn't have much of a roof. He'd been concerned about someone spotting him from the rooftop across the road. But some 'gentle persuasion' of a passerby had enabled him to test the visibility options for himself. That's how he knew he wouldn't be spotted, at least not easily.
In the weeks leading up to the wedding, he'd thrown himself into gathering as much energy as possible, striving to not waste an iota of the stuff. He'd thought long and hard about what he'd do if the opportunity arose and so his plan had come naturally to him.
Sailor Moon and her friends had bested his Youma repeatedly; they'd bested him even. Then, apparently, they'd bested the rest of the Dark Kingdom's upper ranks, including Queen Beryl and Metalia themselves. Since then they'd likely grown stronger, recruiting new members and honing their skills against new, possibly ever escalating, opponents.
So, not only was a direct attack doomed but even taking them out one by one ran too many risks. Jadeite had lowered his ambitions, seeking the most effective way to avenge himself given his new limitations. Eventually he'd settled upon Sailor Moon herself. She, after all, had been the one who'd most frequently thwarted him. On top of that, her friends clearly cared for her deeply, as did she for them, especially Tuxedo Mask. Thus he'd decided to kill several birds with just two stones. Perhaps he couldn't kill all of them, but he could still tear out their hearts.
And to do that, all he had to do was kill Sailor Moon. But that wasn't good enough for him. A quick and painless death was too merciful for his hated enemy; she needed to suffer too. So, first he'd kill Tuxedo Mask. That would break her heart. And, in her shock and despair, she'd never see Jadeite's second strike aimed for her. Finally, he'd strike them down on what otherwise would've been the happiest day of their lives. Even if the Senshi caught him, even if they killed him, he'd die knowing he'd had the last laugh. He might die, but they'd live the rest of their lives tormented by grief, guilt and failure.
However, for his plan to work, for his savage fantasy to become reality, Jadeite couldn't simply mortally wound. He'd need to utterly vaporize his targets. He'd need to be absolutely certain they couldn't possibly survive or recover. That was why he'd needed to gather so much energy; a conventional human weapon would never cut it.
Every night before he'd slept he'd visualized the moment. Then, that very morning, he'd started to make it a reality.