When Isabella went to wuthering Heights after she and heathcliff where married why did Hindley warn Isabella to lock her bedroom door?

Hindley warned Isabella to lock her bedroom door because he was aware of Heathcliff's vengeful nature and did not trust him. He knew that Heathcliff harbored hatred towards him and saw Isabella as a means to seek revenge. Hindley wanted to protect Isabella from any potential harm or mistreatment at the hands of Heathcliff.