Regarding the DM's motives (just or unjust). Maybe DM expects to be but wasn't consulted that it was an evil campaign. Possibly some plot is wasted if the party simply wants to kill guards and rob people. To each their own and sounds like you were doing interesting evil stuff. If the difference in alignment is not enjoyed, it matters who caused this. Maybe the DM is getting revenge for this or trying to level the play field for the lone LG character (and going too far).
Example, I played a pathfinder game where I asked other players that i knew were playing if we could do a good campaign. (I may've said non-evil.) They said yeah so I picked paladin with maxed out diplomacy. I really liked him. Another player joined as evil and turns out everyone wanted evil, but didn't all write it on their sheet. (I wish I knew since we had an evil campaign in same group and it was epic.) Using diplomacy I took too much play time talking to NPCs, and it's avoidable. DM appreciated I was advancing the plot and exploring the world; taking the hooks. The quests turned out fun for everyone. DM was annoyed that other players were not playing alignment and didn't care about the larger world. I roleplayed my character well and so did they, they gave me a proper slaying. I accepted it but could've had more fun.
Reading your story I'm confused by the lack of... Proper Slaying!