I don't think you can justify humanist values, except in some statisticly-pragmatic-for-certain-in-groups sort of way.

Our entire intuition about any form of "good" and "bad/evil" stems from our affinity and aversion to (and judgment of) different "good" and "bad" experiences. But since those judgments all depend on subjective qualia, and since subjective qualia can never truly be shared between subjects (empathy being merely an independent re-creation of presumed qualia in another person), there can be no way of establishing a truly universal judgment on what constitutes good or bad qualia (or, therefore, good or bad actions or events).

There may certainly be certain limited contexts in which certain social strategies may be mostly agreeable to most of the parties present. But when applied universally, any such heuristics will always inevitably be antagonistic to certain subjects, and will always therefore either be egocentric or anthropocentric at best.

Humans are not universally valuable because of their "personhood". Rather, individual humans see themselves as valuable, and most humans are willing (via empathy, or some sort of philosophical argumentation) to project importance onto other humans - at least those humans who they don't revoke the status of "personhood" from (the grounds on which they do so always being somewhat arbitrary or naturalistic — e.g. rejecting a sociopath’s personhood because their behavior or affinities are judged "inhuman").

I've wrestled with this idea a lot, culminating in this article I wrote a couple months ago:

https://medium.com/the-panopticon-publication/morality-is-personal-and-tribal-always-20c8c31f5d29