Roger is evil. In the beginning, he does little things that show he has a dark side:  

Acting on his darker impulses, at first in small ways, he knocks over Percival and Johnny's sand castles. Then he throws stones at Henry, only missing because his arm "was conditioned by a civilization that knew nothing of him and was in ruins."

As the novel continues, Roger progressively grows more evil. When Jack and the hunters trap a pig, Roger tortures the sow. He purposely puts his spear in the anus of the sow and twists until the sow is screaming in agony:

They corner the wounded pig, and when she falls they are on her. Roger is particularly cruel, driving in his spear slowly by leaning his weight upon it until the sow screams in agony. 

Roger shows no mercy. He and the boys laugh at the screaming pig. They become hysterical after seeing where Roger had twisted his spear while inside the anus of the pig. 

Later, Roger pushes a boulder at Piggy and causes Piggy to fall to his death. He shows no remorse: 

Numb to the sufferings of others, he releases the rock that knocks Piggy to his death and tortures the twins into submission. 

By the end of the story, Roger is planning to kill Ralph. He and Jack are chasing Ralph through the jungle. Roger sharpend a weapon with which to kill Ralph:

He prepares a stick, with points sharpened at each end, on which to mount Ralph’s head.

No doubt, Roger is evil He shows no remorse for his evil actions. He becomes more and more dangerous. He is caught up in the game. He kills Piggy on purpose. Now, he hunts for Ralph with evil intentions.