Board Thread:Speculation House/@comment-14138255-20160807005311/@comment-26018514-20160826033712

ChishioKunrin wrote: You guys keep saying that Mercury is in it for the evil or the fun or whatever Cinder might be paying him, but I think he's in it because he feels he has nothing better to do so he might as well....

I would agree with that analysis except we have an example contrary to that ideology in his first kill in the series... it's a sloppy and unprofessional kill he wasn't asked for.

this indicated:

1. He didn't care about risking the mission/getting paid.

2. He wanted to kill.

We also know from volume 3 that he had already proven himself at that point(fighting Amber and before), so it wasn't that, and being at beacon provided him an alternative education.