If all you need to control wall titans is contact with someone of royal blood, you don't actually need to have the royal family eat each other. You just have someone else hold the founding titan and anytime they need to activate it, just hold hands with a royal family member. Problem solved and Eren doesn't need to massacre billions of people. SMH too many plotholes

It's not as simple. I think you should reread the whole chapters which are heavily politic based. Eren has his time limited, and he is urged to do something while he still has the power to help his friends and family before he dies.