Since Astrophil and Stella is a sonnet cycle, understanding some sonnets depends upon first understanding certain others. In this instance, understanding Sonnet 20 depends upon first understanding Sonnets 19 and 21. Sonnet 20, structured with three quatrains, one volta (i.e., turn of thought) at line 9, and a resolving couplet, builds upon the last six lines of Sonnet 19 and is then expanded upon in Sonnet 21.
In Sonnet 19, we learn that Astrophil laments being like Thales who, it is said, fell into a ditch while gazing at the heavens: he combined the deepest depth with the highest heights:
... ... yet what is all
That unto me, who fare like him that both
Looks to the skies and in a ditch doth fall (Sonnet 19)
This means the while Astrophil aspires for Stella's love, he yearns for the highest good yet falls to the deepest despair knowing she rejects his love. With this in mind, let's focus on the "murdering boy," the "tyrant," the "bad guest" of Sonnet 20.
We know that Astrophil, the speaker in Sidney's sonnet cycle, warns his friends to "fly!"because he has received his "death wound" at the hand of the "murdering boy" who shot "his dart" that "pierc'd [his] heart" before Astrophil "could fly hence." Taken in relation with Sonnet 19, Astrophil may be understood as speaking of love for Stella, thus the "murdering boy ... tyrant ... bad guest" with the "glist'ring ... dart" and motions of "lightning grace" is Cupid, the owner of the infamous bow and arrow, the God of Love.
Sonnet 21 opens with another appeal to a friend and reminds this friend that Love--or Cupid, the God of Love--mars his mind as with a "windlass":
Your words, my friend, (right healthful caustics) blame
My young mind marr’d, whom Love doth windlass so, (Sonnet 21)
A "windlass" is a type of winch that pulls, hauls, or lowers, as a windlass lifts or lowers a bucket in a well. So here we have the confirmation of the meaning of Sonnet 20: Astrophil laments that he is both lifted high and brought low in the manner of Thales in his love for Stella because his "bad guest" shot him a "death wound" for Stella with a "glist'ring dart" before he had time to flee from Cupid, the God of Love.