It could be morally good to return some grants if there is a good theory on how this will lead to better results for the people involved and their families, for the communities they are part of (primarily, the EA community), for society, and for the civilisation.
Some deontological motives and considerations could be a part of such a theory. For example, as other people already mentioned in this discussion, returning grants could send a valuable signal to the EA community and to society.
However, it seems to me, the framing of the question "Under what conditions should FTX grantees voluntarily return their grants?" hints at the possibility of some hard-and-fast deontological algorithm for deciding when grants should be returned. I don't think such an algorithm exists. The theories for why returning funds would be good should be far more nuanced, and applicable to very narrow strata of grantees and victims respectively (perhaps even down to individual grantees and individual victims), rather than large strata such as "all grantees" and "all victims", or even "1% of victims who were affected the most in terms of the portion of their net worth that was destroyed".
Considering the above, I think just returning money to FTXFF (or another pool of money) would be ineffective. And even creating a short-lived organisation to administer claims for returns from the victims will be ineffective, too (especially considering the opportunity cost for people who can quickly create locally-effective organisations of this sort: I believe such people have much more valuable things to organise, from the EA perspective).
I think a solution that could be low-investment and also relatively effective is organising a forum where individual victims share their stories and ask for help, and individual grantees can come and respond, assessing their own situation and the situation of the victim, that is, building "a good theory". And then publicising this forum among both the victims and the grantees. This also doesn't mean grantees should return their entire grants, they may help a little, according to their situation and the situation of the particular victim. However, one complication with this solution might be: how could the stories of the victims be verified?
In this setup, grantees should also consider the implications of their decisions for the community and society, not just themselves and the victim. While the latter are highly individual, the former are mostly shared. So it would make sense for some people who are experts in community strategy, sociology, and ethics to write a few essays on this topic that grantees would be advised to read before visiting the forum. (I'm not such an expert.) Of course, individual grantees would still be free to form their own sub-theory regarding these "high-level implications", according to their own understanding of the community strategy and ethics.
Strong disagree. If you are doing high EV work enabled by funding, were not complicit in fraud or other wrongdoing, and are not legally required to, you should not return the funds.
Counterfactually, the funds might have gone to another high EV organization, so you would probably be doing harm by having received the funds and not using it for your high EV purpose.
If you are doing important work, please do not keep that from being done for want of funding where you are not legally required to. EA grantees empowered by funds do incredibly important work and should not defund themselves unnecessarily.