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:

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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.