If people are encouraged (more than discouraged) somehow to stab others, they will, aren't they? Well, kitchen cutlery doesn't incentivize me to stab, except unusual cases. Although, Bitcoin does incentivize me to buy it, sell it, upset the market since I can end up with more money than I had before.
You should also note something significant: If a serial killer starts murdering people with kitchen cutlery, its users aren't affected by this incident. On the other hand, if an individual appears with a lot of money, willing to manipulate the market, they affect the ones who use it ethically*.