Choosing to travel through time to kill a historical figure is easy because we know exactly what will happen if nothing is changed.
Killing a modern day figure is different because we don’t know what’s going to happen in the future. We can guess, but that’s it.
For example, at what point would it have been appropriate to assassinate Smedley Butler? 92 years ago today, it might have seemed like he was poised to become a dictator.