Seneca had a somewhat more humane attitude towards slavery.
See e.g. https://figsinwinter.medium.com/seneca-to-lucilius-47-on-sla...
"Then again, the Stoics were famous for challenging common conceptions, and the founder of the school, Zeno of Citium, had declared slavery an evil in his Republic"
It's important to note that Ceneca and the famous stoics (Marcus Aurelius for example) were rich and benefitted greatly from the status quo.