... the "monstrous god" argument is somewhat slippery because that so often morphs into "any god is monstrous who isn't the way I imagine God to be" (i.e. as in "only a monstrous god would allow Hell/evil/suffering as the coin of the realm, etc...)".
So here's what's monstrous about a god who would accept my damnation for my brothers' salvation:
First, I have in mind a god, along with that god's revelation of salvation and damnation, that is as close as possible to reality. How much, or rather how little, would a god who would accept the damnation-for-salvation offer have to differ from the God Who Is?
Well, the God Who Is is both Justice and Love, and there's nothing either just or loving in being prepared to accept such an offer. Such a god would not particularly will my good, assuming it's good to be saved -- if he did, then it would be a sin for me to offer to be damned. And to punish one person in place of another is not to render to each what is due them (and again, this differs from Christ's sacrifice not least because it deals with eternal life rather than temporal life).
Moreover, how could we reconcile the possibility of damnation-for-salvation with the Two Great Commandments? Only by making loving God with all your heart, soul, mind, and strength consistent with separating yourself from Him for all eternity as a means of loving others. If that sort of inversion of the commandments is possible, do we really have a god lovable enough to warrant loving him as the greatest commandment?
For that matter, the second greatest commandment is to love your neighbor as yourself, and damning yourself for your neighbor is loving your neighbor even more -- unless, again, damnation is good for you, and I don't think even the staunchest double-predestinarian would say it's blessed to be damned for loving God and your neighbor.
And what would be the practical effect of the damnation-for-salvation possibility? We would all, each of us individually, be obliged to ask this god to damn us for the sake of our brothers. It would, after all, by assumption be in accord with the god's will and with love of neighbor, so how could we fail to do so?
In sum, we'd have a god that doesn't love us demanding us to love him, then allowing us to do in a way that entail eternal punishment. Unless the whole thing was merely a Catch-22, and whoever offered to be damned was saved and whoever didn't wasn't -- which would be even more perverse.