Why would Lelouch be haring off after the cult if it was Suzaku? If Rolo were innocent, he would at least have seen the killer, being a trained assassin and all. What is his motivation to blame it on the cult if Suzaku killed Shirley?

While I think there's more to this than just Rolo shooting her in a fit of jealousy, Suzaku has no motivation to shoot her and is too good of a soldier to do it by accident. Let's try finding some actual evidence before pointing fingers, hmm?