There is no such defence against a charge of genocide.

The lawyers who wrote the international treaty, many of whom themselves survived the Holocaust and lost their relatives in it, carefully considered whether to add such a defence. They did not add it. They considered that genocide is a crime for which there is no excuse. That is should be possible to defend yourself without resorting to it.

In any case, the group at issue is not Hamas. The genocide is being conducted against all Palestinians.

Your argument also conveniently omits the extreme level of military dominance which Israel has over the Palestinians.

The real reason many Israelis cannot conceive of a solution other than killing or expelling them, is: how can we leave them there, after the level of hatred, murder, violence, and abuse we have heaped on them over the last two years? We have taken revenge for our 36 dead children, won't they want revenge for their 20,000?