Bheema harassed Duryodhana and the other Kauravas. He would grab the Kauravas bathing in the river and hold them down under water till they cried for help.
When the Kauravas climbed up a tree, Bheema would uproot the tree and shake it vigorously. The Kauravas would fall off the tree like ripe fruits.
Duryodhana was jealous of Bheema.
He often thought, "I must do something to prevent Yudhishthira from ascending the throne of Hastinapur. But I cannot do anything as long as the mighty Bheema is with him. So, I must eliminate Bheema."
Duryodhana would think of many evil plans to kill Bheema.