Remember what Reg says in Douglas Adams' Dirk Gently's Holistic Detective Agency? He claims he's tried to fix some similar problems in history by using time travel, but that those attemts have always backfired and have turned something else much worse.
Remember this conversation of Harry and Dumbledore in the Prisoner of Azkaban?
‘But – I stopped Sirius and Professor Lupin killing Pettrigrew! That makes it my fault, if Voldemort comes back!’
‘It does not,’ said Dumbledore quietly. ‘Hasn't your experience with the Time-Turner taught you anything, Harry? The consequences of our actions are always so complicated, so diverse, that predicting the future is a very difficult business indeed … […] You did a very noble thing, in saving Pettigrew's life.’
If Harry is effectively helped Voldemort rise again with what seemed like a noble action, would you dare to go back in time several decades to kill the young Riddle and bear all the consequences of your action? Could you be sure you're acting for the better in the end?
Remember what Gandalf answers when Frodo asks why he ordered to keep Gollum captive instead of killing him?
“[...] He deserves death.”
“Deserves it! I daresay he does. Many that live deserve death. And some that die deserve life. Can you give it to them? […]”
You have to be very careful before you demand to kill people for their sins.