There is a kinship bond within members of the same species, although it isn't universal. As John Donne said, "Any man's death diminishes me, because I am involved in mankind; and therefore never send to know for whom the bell tolls; it tolls for thee." We're human and the Sentinelese are human, and even though we're not genociding them (mostly, I think, because they have nothing worth killing over otherwise they would have been dead a long time ago,) we're still genociding plenty of other fellow humans. We even avoid killing animals to extinction because humans share and suffer the consequences of living in a complex ecosystem with other beings. But we also cut the fins off of sharks for soup and throw them back into the water to drown.

But none of those connections would exist between aliens and humans. We're not one of them, nor do we share the same biome, so our deaths might well be entirely void of meaning or consequence for them. In a Dark Forest scenario, there's little to no advantage to contact or cooperation with an alien species, and every possible advantage in sending a GRB to fry anything that so much as looks like life before it has a chance to evolve into a threat.