To help a tyrant do evil — that is what 'aiding Jie in his tyranny' means.
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history
Jie was the final king of the Xia dynasty, traditionally remembered as a debauched tyrant overthrown by Tang of Shang. His name pairs with the Shang tyrant Zhou (纣) in the set phrase 桀纣, meaning 'cruel despots.'
usage
In classical texts 桀 also means 'outstanding,' interchangeable with 杰 (hero). The compound 桀骜不驯 ('unruly and untamable') is the most common modern use of this character.
Bottom 木 is the wood radical, the original wooden roost-post the feet stood on. It is the indexing radical, though the modern tyrant meaning has nothing to do with trees.
Top is 舛, two feet pointing apart ('to oppose'). Originally the graph pictured a person standing on a wooden post — feet up on a perch above the tree below — giving the early sense 'roost-post,' later borrowed for the king's name.
