We include ability cards to help ensure that key information is shared and they can make the games flow. But you don’t have to use them, as shrimpy47 explains:
I've been playing murder mystery games for 20+ years. In the beginning, boxed games. The last 10 yrs or so, freeform games. These are the best! I don't like the idea of staging fights or killings all night long, so I just eliminate the ability cards and allow my guests to do what they want. I've had many guests plan and murder other guests without having to use a card, or ask the host permission. No one ever picks a fight and I don't think it takes anything away from the game. We always have a blast playing these games! The only rule I make is that the only people that start the game with a weapon, are those that have that item (item card).
It's amazing during the game how many people will acquire someone else's weapon by bribery, blackmail etc. We also all enjoy having the items instead of the item cards. We usually inexpensively make them or purchase them. The guns are colorful squirt guns and the knives are either made out of foil or a rubber child's toy. Many items are simple, and can be found around your house; a book, piece of rope, a note, coins etc.
At the start of the night the guests are given a large manila envelope with all of their information, money and items. Making these changes with every game we've played always works and we have a great time! We've played 6 of these games already and can't wait to play the next one!
In a similar vein, Ursula Zwick suggest: If you're concerned that your players will throw themselves into the game too quickly, you can forbid use of ability cards for the first 20 minutes – to give everyone a chance to at least talk to each other first!