This is a two parter.
1. Why would the leafs greatest there hokages make a seal the contained all there forbidden Justus? I mean that just screams! Come take me I'm filled with lots of powerful Justus!!!!
2. For them to be known as good people. Why did the second hokage make a jutsu that requires a sacrifice? Killing someone just for a jutsu sounds bad right? So why would be make a jutsu that needs a sacrifice?