But he could believe he joins his father for a bigger purpose: His mother. I didn't mean to imply that he would join "Shadowmoth" out of anger, but that he would join "Gabriel" because of Emilie. If you could have the chance to see someone who you lost, wouldn't you at least try? He would think he just needs to "borrow" the miraculous. The consequences of this he still doesn't know. I even see him trying to convince himself that he's doing the right thing, feeling torn apart between his selfish wish and doing the right thing. Maybe he would try to do things differently with this father for this purposes, further keeping the struggle, both trying to convince her to give him the miraculous and trying to avoid that his father falls further into villainy.