Gan Ning was not a backstabber - he only ever betrayed Huang Zu, who had treated Gan Ning horribly and attempted to steal his soldiers. To suggest he was aiming for Ling Tong is a bit preposterous.
Gan Ning simply did what he felt was his duty as an officer of Sun Quan - he saved the life of an ally. Perhaps he also intended to make amends with Ling Tong.
Looking at Gan Ning, we should look at various sources in finding his personality. On one hand, we see a brazen brute, more than eager for the opportunity to swing his sword around. On the other, we see a very civilized and at times sensitive individual, as is evidenced by his civil attitude towards Sun Jiao when Sun Jiao drunkenly insulted him.
"There are those who try to shape the world to their own whim,
and then there are those who allow the world to shape them.
It is in the balance that greatness is achieved."