John Gill Commentary Luke 3:20

"added this also to them all, that he shut up John in prison." — Luke 3:20 (ASV)

Added yet this above all
This sin to all other sins, and which was of a more flagitious nature; and attended with more aggravating circumstances, especially in the issue of it:

that he shut up John in prison ;
in the castle of Machaerus, by the instigation of Herodias; (See Gill on Matthew 14:3).