Current assumption is either Monique because she is secretly evil, or Zoe's parent's who are clearly part of the Saintess faction and want to gain influence off their daughter's rise to power as an Apostle/Saintess. Laz will probably be revealed as been Ciro's friend during the last selection exam and has been trying to help Chloe.
Since it seems like the 2 final Apostles end up as 1 Gifted and 1 Non-Gifted, the only real reason to kill Ciro was if you had a Gift and were out shined by them, or if you didn't have a good relationship with them and weren't going to be paired up with them at the finals. Assuming Monique may be secretly evil in a [she grew up poor but learned that she was Gifted in manipulating people and decided that she deserved everything she wanted] way, she could have framed her gift as animal communication and when she got to the selection exam and found someone who had the "same Gift" and was older and more qualified, she had to eliminate her to gain a better position. On the other side, if Zoe's parents were informed that she didn't have a close enough relationship with Ciro who was frontrunner as the Gifted Apostle finalist, they may have determined it would be better to eliminate Ciro and have Monique selected as the Gifted Apostle since Zoe had a better relationship with her.