
Because that shadow grows stronger everytime it killed someone, or it came across a new corpse. Remember what is MC's skill again? Destiny stealing. Which he mainly use on dead people to gain their skills. The more destiny he gains, the more powerful he becomes. Though each time he uses it, his karma increases.
Now, the shadow has two characteristics. 1) it can use the original's power. 2) the more karma the original has, the power the shadow has. The more it uses "destiny stealing", the more power, skills and karma the shadow gets. So you can imagine what sort of monster that shadow is.
wait i don't get that last part of ch45