First of all, Zhou Yu and Zhuge Liang did not want to slay Cao Cao, nor did they want to make enemies with Cao Wei and recruit soldiers. Battle of Red Cliffs and Cao Cao were defeated, but both Sun and Liu were obliged to kill Cao Cao, but neither side wanted to. The biggest reason is that they don't want to cause more disasters and troubles. No matter which side slays Cao Cao, it will have hatred with Cao Shi Group for generations, and then Cao Cao's sons will do their best to avenge their father and lead the army to conquer. Neither Sun Quan nor Liu Bei wanted to offend the most powerful Cao Wei.
Therefore, Zhou Yu deliberately surrounded on three sides, leaving only one side as an exit. This is the direction of Liu Bei's garrison. Zhou Yu also wanted to kill Cao Cao by Liu Bei's hand, thus leading the disaster to Liu Bei. Zhuge Liang is also a wise man, so naturally he will not slay Cao Cao. As can be seen from Zhuge Liang's deployment, he first asked Zhao Yun and Zhang Fei to ambush Cao Cao, but both of them killed Cao Jun, robbed the horses and other weapons, and did not pursue Cao Cao. Therefore, Zhuge Liang wants to kill Cao Cao, and he may not have to wait until Huarong Road. Even Cao Cao himself saw that although Sun Quan and Liu Bei were chasing after him, they didn't want to kill him. So judging from the situation at that time, Zhuge Liang could not slay Cao Cao.
Secondly, Zhuge Liang didn't kill Cao Cao because Cao Cao couldn't die at that time and the world still needed him. Zhuge Liang Longzhong made it very clear that Liu Bei wanted to occupy Jingzhou, conquer Yizhou and capture Hanzhong, so that he could leave Qinchuan and destroy Cao Cao. However, after Battle of Red Cliffs, there were only 30,000 soldiers of Liu Bei, who would not cross Zhang Zhao. Only Jiangxia Town had a foothold. Looking back at Zhou Yu, sitting in Jiangdong, Liu Bei is no match at all. Looking at Cao Cao again, although it failed, the foundation in the north is still very solid, with hundreds of thousands of troops, horses, money and food, and the largest tax population. The north is still the political and economic center of this country. Cao Wei is still the best of the three parties.
And if Cao Cao is killed, the north will fall into chaos again, and Liu Bei's tens of thousands of troops will no longer be out of control. Sun Quan of Soochow will not let Liu Bei go, because Liu Bei is likely to become an enemy of Soochow, and Sun Quan also has an emperor's ambition. At least Sun Quan won't give Liu Bei any chance to be a servant in Jingzhou. And Liu Bei will be defeated and flee under the attack of Cao Wei and Sun Quan.
Thirdly, if Zhuge Liang sent someone to slay Cao Cao in Huarong Road, Liu Bei Group would never have a chance to plunder the fruits of Battle of Red Cliffs's victory. As soon as Cao Cao was killed by Zhuge Liang, he would regard Liu Bei as the number one enemy in Jiangdong and lead his troops to crusade against Liu Bei. Sun Quan won't give Liu Bei a break and let him take Jingzhou. We know that it is precisely because Cao Cao was not killed that Soochow naturally regarded Cao Cao as the number one enemy.
Zhuge Liang also took advantage of the decisive battle between Zhou Yu and Coss in Nanjun and ordered Zhao Yun to attack Jingzhou. Liu Bei took advantage of the situation and captured Lingling, Guiyang, Changsha and Wuling as a foothold. It was because of the pressure of Cao Wei that Sun Quan agreed to let Liu Bei stay in Jingzhou for the time being, and there was no crusade. Imagine if Cao Cao of Huarong Road was killed, would Liu Bei still have a chance to seize Jingzhou? Therefore, considering various factors, Cao Cao could not die after the defeat in Chibi, so Zhuge Liang sent Guan Yu to guard Huarong Road and let him go.