Sebastian and Antonio clearly are presented as ne'er-do-wells through the way that they constantly mock Gonzalo and make fun of him in this scene, sometimes very cruelly. Given that Gonzalo is presented as a faithful retainer who is loyal, wise and brave, this making fun of Gonzalo indicates that Sebastian and Antonio are completely the opposite to Gonzalo's character. Where Gonzalo is faithful, they are unfaithful, and where Gonzalo is wise, they are unwise. Note, for example, how they mock Gonzalo's ruminations on what he would do if he was in charge of the island:
GONZALO: Had I plantation of this isle, my lord--
ANTONIO: He'd sow't with nettle-seed.
SEBASTIAN: Or docks, or mallows.
Such interruptions from Antonio and Sebastian speak of ill-breeding and childish mockery. However, far more disturbing is the way that Sebastian points out to Antonio that Gonzalo would have this island as a place free from "sovereignty" whilst at the same time wanting to be King of it. Such an observation is particularly interesting given Sebastian's own desires to kill his brother and seize the crown.