How many people would cease to exist due to your meddling? This is why we have the Temporal Prime Directive.
I would love to travel to next week and record the winning lottery numbers. When I came back and bought a ticket, within 7 days, what I had seen on my travels would therefore be wrong, since I should have seen myself winning the jackpot. Very confusing.
If you got the winning numbers...... From the future...... And there was a winner...... You would have to split the winnings? Is that pessimistic?
Whether or not changing the past is ethical might make an interesting discussion and thread in its own right.