Please, consider the following.
Hint: the (scientific) name of the flower is Strelitzia
Strelitzia is the boss mech
LAURIAM DID NoThING WRONG
Let's stop assuming he murdered his nobody mech scythe friend person
But seriously, here's some more eloquent explanations:
As we can see there is a clear connection because our dead keyslinger and Marluxia's boss form, which is a hooded woman with a clear design inspired by the Strelitzia flower. I propose that this indicates a familial relationship, not one of murderer and victim. Marluxia having killed Strelitzia is far too obvious in general, and is pretty clearly a red herring on Square's part.
However, this doesn't mean Marluxia had nothing to do with her death, Lauriam likely indirectly cause it, by say, telling someone he shouldn't have about being a new union leader, the world ending, or a similar situation. I would like to say that one murder does not an 'assassin' make, and he likely earned the epithet from unsavory work during the keyblade war, or on a warpath for vengance against anyone involved in Strelitzias death.
The female figure in Marluxia's final form seems to be protecting him, and doesn't seem to be an enslaved creature at all, though I know that is still technically a possibility. Given the new splash art for JP KHUX- where the org coat clad hooded figure is clearly Strelitzia, we already know she is a nobody- possibly the first ever- beyond a shadow of a doubt. Her returning or being sustained by Lauriam/Marluxia to help him doesn't seem that far fetched.
Strelitzia's own moral complaint was that they were not able to save everyone, that they were basically choosing who 'got to survive the Apocalypse', and effectively playing God. A sweet girl like that helping someone as morally ambiguous as Marluxia may not be so far fetched if she has already been advocating for those with darker hearts to be saved.
Friendly reminder that technically, dastardly methods or otherwise, if Marluxia succeeded at the end of Chain of Memories he would have put a stop to Xehanort's plans and quite possibly saved the universe.
