Someone that either died in an accident or got murdered.
For instance if I were going back to March 31, 1995 and somehow stopped this crazy fan (by probably stealing her purse and showing her the gun she was about to use) from shooting former singer Selena Quintanilla.
If we had the ability to do that, would the future alter for the better or worst.
Most Helpful Guy
The Butterfly of Doom would seek vengeance. The changes would be mostly bad. I even demonstrate this in a Camelorum Adventures crossover fanfic. Carly (locked up in Camelorum for having uncontrollable randomness powers) gets Percolated along with Jenny Jane to S1Ep10 of Orange is the New Black, and they try to save Tricia. However, their antics only result in her becoming that much more of a martyr, as well as turning Menendez into a Sith lord. Oops. The Percolation punishes them by trapping them in a 1980s dom com, where they meet a blatant knockoff of ALF.
They're extra careful in the 80s not to step on too many butterflies. Though, they do visit a Fretter store after Belf and Geoff take them to one. When they find themselves back in their native universe, they're happy - even though it's still Camelorum. And the weirdest thing they have to worry about outside the walls of Camelorum are Accel the Aardvark and Dr. Bamboolid fighting again. At least the Judo Iguanas are keeping quiet.1
Most Helpful Girl
It would be negative. Death is simply a part of life. It's a natural occurrence and going back in time to change just one thing like that would obviously impact the future in a big way.1