I have tagged posts about Walker as proslavery, but that puts my thumb on the scale a bit. My sources, both older and more recent, do not treat him as a consistently proslavery figure. In The Impending Crisis, David Potter writes:
Walker’s experience also offers an insight into the relationship between filibustering and slavery. The Man of Destiny was, of course, from a slave state, and he accepted slavery as a matter of course, but there is no evidence that he was dedicated to the expansion of slavery, and the impulse of some historians to picture him as a minion of the “slave power” reflects a failure to recognize that Walker may have been exploiting the proslavery elements, instead of their exploiting him. In September 1856, with defeat staring him in the face, Walker revoked the decrees of the former Federation of Central American states which had abolished slavery in Nicaragua, and in 1860, in his book The War in Nicaragua, he pictured his republic as a potential field for the expansion of slavery. But in both cases, it is clear, he was trying to win desperately needed support for his own personal rule in Nicaragua. Until this need arose, his history had been simply an adventure story, a drama of daring and conquest to fulfill the glorious destiny of a superman rather than to serve the interests of a section.
William W. Freehling agrees in The Road to Disunion: Secessionists Triumphant:
This latest casualty not of Yankee presidents but of Latin executioners was only occasionally more proslavery than López. William walker preferred free soil California, his most common American residence in the 1950s, to enslaved New Orleans, where he restively spent the late 1840s. The “Gray-Eyed Man of Destiny” never held slaves, never farmed, never married, never owned land, never settled in any profession, never stayed in any community.
The shy dreamer, always wrapped (even on the battlefield) in an enormous preacher’s frock coat, never preached a proslavery syllable in his conquered land or legalized Nicaraguan slavery until his hold on the country had slipped, a year after he seized power. His speeches down south on proslavery Caribbean adventuring came only after his support in New York and San Francisco capitalistic circles had dried up. […] His first soldiers were failed gold dusters, not enterprising slaveholders. His Nicaraguan army ultimately enrolled as many foreign-born was southern-born troops and more northern-born soldiers. His men were scarcely ever slaveholders, rarely farmers, usually the poor young sports in the cities where capitalistic merchants financed his flings in Nicaragua.
Faced with those facts, I don’t see Walker himself as a proslavery diehard. He comes off more as a restless glory hound and opportunist, with most of his men falling under the same category. But I hold that his expeditions amounted to proslavery efforts, even if they did not begin with that specific goal. Certainly many of Walker’s financiers and admirers saw them as a way to expand slavery. As a Southerner quite comfortable with slavery, would Walker stand in its way if Nicaragua remained his and Southern settlers rushed in and demanded slavery come with them? Possibly yes, as some Southerners did oppose slavery and seek its end, but his lack of past commitments on the matter hardly argues for that outcome. The same pragmatism that led him to wrap himself in slavery when he came to dire straits would almost certainly prompt Walker to do the same to consolidate a more successful regime. Walker would get a proslavery result from political calculation, since his own apparent disinterest in the subject would hardly motivate him to swim against the current.
One can also read the timing of Walker’s repeal of Nicaragua’s abolition differently. When Walker first seized power, of necessity he had a coalition with the Nicaraguans he joined with on arrival. He had to share power with them and, to some degree, the Nicaraguans he defeated as well. That may have limited his ability to act on any proslavery impulses he had at the time. When they deserted him, Walker finally had a free hand on top of his desperation. I can only speculate on that and must defer to his biographers, but it at least seems reasonable that as Walker took slavery for granted, he could take its introduction equally for granted. He certainly did when he seized Lower California and adopted Louisiana’s laws for his new Republic.
I emphasize, however, that I only speculate in the previous. As I wrote a while back, when I disagree with the experts readers should probably trust them instead of me.