Quoted By:
Yes it's poorly written however he did say he lost control of it. Reading between the lines it seemed like he thought they'd be able to control it but it went berserk and escaped. Classic mad science shit.
I doubt they thought about this while writing it but how about this for a head cannon. Rose was pissed Leon was going to blow him off to go have fun and have pokemon battles. Maybe he didn't think leon would even actually stick around the next day and help him with his project. He needed Leon there just in case the project went sideways. So he did it in the only time he knew for certain Leon would be there and prepped for battle.
He didn't need Leon who had all of his pokemon injured, he didn't need Leon who was out of sorts after his first loss and sad over losing his title. He needed Leon the undefeated champion.
