Avenging the Loss
Under almost any circumstances, Cao Cao would have spared Lu Bu's life. Lu Bu had served under some of the most fearsome tyrants of their time and lived, though he had gotten personally close to them. He had also killed those tyrants. That gave a man a certain reputation. During his time as a leader, he had strategized well, but exercised poor discipline over his troops. A definite flaw to be corrected. If Cao Cao read him correctly, Lu Bu wanted people to respect and fear him and didn't particularly care about soldiers under his command. That would have suited Cao Cao's purposes well. He could have kept Lu Bu as a secret weapon, a solitary force to unleash when the time was right, like a battering ram.
Yes, to save Lu Bu's life was tempting, all except for one thing: Lu Bu was responsible for Xiahou Dun's loss of an eye.
Cao Cao gave the order to put Lu Bu to death.