Although the main theme in "The Highwayman" is mainly love could it also be despair that caused them to act so drastically?
This is an interesting point to consider. However, I think that if we look closely at the poem, we can see that actually see that despair really had no place in the actions of Bess and her love, the Highwayman himself. This is because their actions, although definitely drastic and resulting in their own death, are shown to be done out of love for each other rather than through any despair. Note how Bess is described as she kills herself to alert her lover to the presence of his enemies:
Nearer he came and nearer! Her face was like a light!
Her eyes grew wide for a moment; she drew one last deep breath,
Then her finger moved in the moonlight,
Her musket shattered the moonlight,
Shattered her breast in the moonlight and warned him—with her death.
Bess's face is described as being "a light" due to the nearness of her lover. Her death is shown to be an act of self-sacrifice that is done out of love rather than any sense of despair. Likewise, too, the Highwayman's last mad rush into his enemies is done in a fit of anger out of the love that he has for Bess rather than anything else. Thus despair does not seem to be a part of this excellent poem.