Theory:

     “Who hath dared to wound thee?” cried the Giant; “tell me, that I may take my big sword and slay him.”
 
     “Nay!” answered the child: “but these are the wounds of Love.”
 
     “Who art thou?” said the Giant, and a strange awe fell on him, and he knelt before the little child.
 
     And the child smiled on the Giant, and said to him, “You let me play once in your garden; today you shall come with me to my garden, which is paradise.”
 
     And when the children ran in that afternoon, they found the Giant lying dead under the tree, all covered with white blossoms.
Explanation:
 
The giant couldn't tolerate the little kid being hurt. After all, he was the favourite of all children he had seen in all those years. He was the beloved kid who stole the giant's heart forever. He was the one who brought love and mercy upon the giant's heart. So, he no longer waited but shouted angrily, enquiring the little boy as to who made those wounds on his little palms and feet. Then, he said he would kill that person with his sword who have hurt the little kid.

The cute little kid replied that it was not a mark of injury but a wound that indicated love. When the giant asked the little boy as who he was? A strange awe fell on him.
 
God's invitation to paradise.jpg
Christ as the little boy
 
It was Jesus Christ himself who appeared in his garden as a little boy before years.
 
Christ inside the giant's garden.jpg
Christ inside the giant's garden
 
When the little child revealed his identity to him, the giant knelt before him. The nail prints on the hands and feet remind us of Jesus Christ, who got crucified for the sake of saving us from worldly sins. Similarly, when the giant was selfish, it was the little boy who brought love inside his heart and made him realise the importance of selfless love, thereby saving him from the sin of selfishness.
 
The little child smiled at the giant. He told the giant that since he allowed him to play in the garden once, now he came to take him to his place, which was paradise.
 
Christ among children.jpg
God among children
 
God gave him space in heaven as a gift of his selfless love he showed on the children all the years.
 
Christ as the little boy .jpg
God inviting giant to the paradise

The giant was found to be dead when the children entered his garden that afternoon. His body was covered with white blossoms under the tree. White flowers symbolise 'Purity'. As God was happy about his selfless life in the world, he gave space for his pure soul to rest in heaven.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
SlayTo murder
2.
NayNo
 3.
Awe A feeling of great respect and admiration combined with fear
4.
ParadiseHeaven
5.
Who art thou?Who are you?
Reference:
National Council of Educational Research and Training (2008). The Selfish Giant – Oscar Wilde (pp. 17-24). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.