Can I use free will as a defense in my murder trial?

I'm an armed security guard and I'm being charged as an accomplice to 1st-degree homicide because I stood by and watched a guy shoot five people without doing anything. I was going to plead stupidity, but I figure I'd be better off if I said that I just didn't want to intervene because I would be depriving the guy of the free will to murder. I think it will work, and even if it doesn't, this will only make my legal stupidity defense stronger, won't it?