Maybe either Cecil or Yzma tried to kill the other, and a third person jumped in to finish off the... the survivor?

[Ugh, talking about this just puts a seriously bad taste in her mouth. She was fine playing investigator when it came to Atilla, but actual murder...]

Whoever built the catapult could've been the one who tried to murder the other first. I don't think a third person would've been able to build that while other horrible things were going on close by.