Who said I was a shape-shifter? I just said that I can acquire powers and abilities.
Well you said if you took the killing power of the mafia, you would turn up mafia when investigated.
In the end your 'ability' to acquire powers could come up to harm the town, and whilst you may not be against the town now, who knows that when we get down to minimal numbers you aren't going to stay with the town?
Maybe you suddenly turn against us and poof, you have won.
There isn't much point keeping someone around who isn't town, you're just another block in the way of us winning. It's not what you will or won't do now, it's what you might do in the future and having played in other games where this has happened it's not a risk I want to take.