the sinless One became sin for us

And the Lord said unto Moses, Make thee a fiery serpent, and set it upon a pole: and it shall come to pass, that every one that is bitten, when he looketh upon it, shall live.
And Moses made a serpent of brass, and put it upon a pole, and it came to pass, that if a serpent had bitten any man, when he beheld the serpent of brass, he lived.
Num 21:8-9 KJV

The figurative interpretation of this Scripture is that “the fiery serpent” [bronze serpent] on the pole represents Christ on the cross.
Without exception, the serpent in the Bible is a figure representing evil. From its role in the Garden of Eden to the dragon’s defeat in Revelation, the serpent is the embodiment of evil. How, then, can “the bronze serpent” on the pole represent Christ?
God made Him who had no sin to be sin for us, so that in Him we might become the righteousness of God.
2 Cor 5:21 NIV

For our sake God made Christ virtually to be sin Who knew no sin, so that in and through Him we might become the righteousness of God.  Through Him we are approved and accepted, and in right relationship with God.

The suffering of the sinful nature came upon mankind as a result of the ministry of the serpent, the penalty and immediate sentence was death!

But, the sinless One became sin for us, and nailed our sins to the cross. Therefore, all who look to the cross will find their sinful nature nailed to it—and they are resurrected with Christ to New Life free from the corruption and curse of sin.