I'll agree that his rush to the combat is almost certainly an attempt to keep Harry alive.

When going out of your way to turn a troll into an assassination tool, wouldn't you also instal some explicit instructions NOT to harm Harry Potter or anyone else vital to your future plans? At the very least the troll was ordered (imperio-ed?) to ignore any other victims and go straight for Hermione since it let Filch go. The fact that after killing his target the troll attacked anyone nearby makes me suspect Lucius, who has no reason to keep any Hogwarts student alive since his son is no longer there.