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.
This critique applies only to unspent funds that have not been irretreviably committed and only applies to certain factual scenarios.
I don't think we have enough information to evaluate this position. In some plausible narratives, the grant funds were straight up stolen from the depositors and given to the grantees. In other words, most or all of the "profits" could just be misappropriated client funds. The fact that FTX handed over the money to grantees doesn't change that.
If someone steals my car and gives it to their friend for free . . . it's still my car. The friend is not morally free to decide they would derive much more utility from the car than I would (or realistically, than my insurance company would from not having to pay the claim). Knowingly retaining my car without having provided appropriate value in exchange is just another form of theft.
In those scenarios, the conclusion that the victims should bear the loss leads to a conclusion that it's OK to retain knowingly stolen funds if the cause is good enough and/or that it is OK to force people to involuntarily donate if the cause is good enough. In my book, it is tantamount to theft -- the grantee is acting to continue the initial deprivation by the grantee's confederate of the victim's property
If we think it is OK to reduce Joe Crypto's savings to 50K to avoid disrupting the work funded by FTX grants, have we all reduced our savings to 50K first? I can't figure out a world where it is morally appropriate for a grantee or EA community member to decide it is OK to involuntarily appropriate Joe's property on utilitarian grounds without having made an equivalent level of financial sacrifice themselves.
I am aware of the dangers of "you're not donating enough" type logic. But EA is supposed to be impartial; we are not entitled to demand Joe Crypto make sacrifices for the cause we are not willing ourselves to make. So I think it is an appropriate question to ask here because we are potentially talking about involuntarily taking the money of someone who is less well off than we are without making an equivalent sacrifice ourselves.
Again, there are a number of scenarios in which I don't think there is a moral obligation to return funds. In particular, if the grant was clean when made, any future bad acts by the grantor can't retroactively render it the equivalent of stolen property.