Lelouch isn't so much using Nunnaly as a crutch as Nunnaly is holding him back.
Lelouch truly cares about revenge against the royal family, and all other things a secondary. However, he is still human, and as such he still retains a conscience. He cares for Nunnaly and hasn't descended into complete bastardry because he was being HELD BACK by himself, trying desperately not to become a man like his father. If Nunnaly were to die, Lelouch would be empowered with more anger and would no longer have anything preventing him from doing whatever it takes to destroy his enemies, sacrificing whoever it takes to do so.
Suzaku however, was always the type of person who was defied by his conscience, always thinking about what was "right" and never once thinking about himself or his own needs. Suzaku is far more shallow a character than Lulu is, by virtue of the fact that his motivation hasn't changed, despite the fact that his temperament may have.