Destiny Bond's effect lasts until the beginning of your own Pokemon's next action, and wears off at that point. If you faint within that period of time, and another Pokemon's damaging attack was what dealt the finishing blow, that Pokemon will lose all of its HP. So using Destiny Bond and following up with Self-Destruct in an attempt to force Destiny Bond to trigger that way won't work for two reasons: the Destiny Bond effect is removed at the very beginning of Self-Destruct's execution, and even if it wasn't, Destiny Bond's revenge mechanic doesn't apply to self-inflicted HP reduction (if it did, what would it do? Get revenge on itself and make itself super-duper fainted?)
There is of course a chance that the raw damage output of Self-Destruct is enough to get a KO normally, but Gengar's unimpressive attack stat makes that a longshot, and if that happens then you didn't need Destiny Bond at all.
