John Gill Commentary Mark 6:19

"And Herodias set herself against him, and desired to kill him; and she could not;" — Mark 6:19 (ASV)

Therefore Herodias had a quarrel against him

Was angry with him, conceived wrath, and laid up hatred against him, and was an adversary to him; which is the sense of the Oriental versions. She bore him an inward grudge, and lay in wait to take the first opportunity of dispatching him:

and would have killed him, but she could not ;

being hindered by Herod, who as yet would not suffer it; partly through fear of the people, and partly through the respect he had for John.