The plot is scripted that Malak ultimately turns Bastila to the Dark Side, the force bond with Revan helps in weakening her resolve. So in a way I suppose you help her fall to the Dark Side just by sharing a force bond.
Darth Revan if DS can persuade Bastila to join him as his apprentice if you have a high influence with her.
Light Side Revan can help Bastila return to the path of the light.
Oh ya, i completely forgot about the end of the game. I was kinda wondering if you could turn all of your players to the dark side cause in number 2 you can make all your players jedi thats what got me thinking.