I will withdraw, but this intrusion shall/ Now seeming sweet, convert to bitterest gall.” what does tybalt mean by this

He won't let Romeo get away with gate crashing the Capulet feast. Although he does put his weapon down, he does do so reluctantly, under his uncle's orders. He threatens revenge upon the young Montague. This, of course, foreshadows the tragic future of Romeo and Juliet (deaths of the two protagonists) as the Prologue at the beginning of act 1 states.

Hope this helps.