I think an Mc has to be likeable, or at least relatable. Likeable is better. fter all, this is the person whose journey we are along for the ride with, and the outcome is going to affect them. If we don't care about them, why will we care about the ending.
If we take the serial killer...
As a reader, I don't like it if the protagonist dies. It's pretty much a ticket directly to the 'this book sucked and I will never read it or another book by this author' again pile.
It's OK to kill secondary characters. The more secondary, the more acceptable.
It's even better if you...
I won't buy a $0.99 priced book. On the subconscious premie that you get what you pay for probably, combined with a few buying experiences that didn't contradict the theory.
Tobias Buckell did an informal experiment with his book of shorts and found best sales at $4.99. Looks like Michael...
If you want to really make up a NEW expletive you need to think about what's sacred and what's reviled in your world. In our world we swear by reference to what's sacred (god, jesus, etc.) and by what's reviled (shit etc.). So you either debase something or you compare someone to something vile...