If you could go back in time and lets say kill Hitler before he raised to power, you wouldnt exist to even do it most likely.
A lot of people today exist because of WW2, case in point all the baby boomers.
SO lets say you kill Hitler, way before he gets to power. Well, first your grand parents may never meant or be into a position to meet. This would lead to you or your parents to not exist, meaning you wouldn't exist.
Along with all the technology that came out of WW2, most of that wouldn't exist until later or not at all.
So if you could go back into time, you don't want to interact with anyone or anything. The smallest thing could change the world for ever.
I would want to go back in time and witness the birth of the United States, witness my founding father create the US constitution, as this is one of the most important moments in human history. But I wouldn't interact with them at all for fear of changing the future.