[stouset]

By the time you know whether or not another civilization is hostile, it’s probably too late. “Interstellar war” sounds like a long dragged-out set of engagements when it’s more likely that one random day without warning your planet intersects a sizable chunk of tungsten traveling at 0.9c.

Better to strike first than gamble.

[perilunar]

Another reason not to confine your civilisation to one planet, or to planets generally.

Much harder to wipe out a civilisation that's dispersed among hundreds or thousands of smaller space colonies. Especially if many of those colonies are hidden in an asteroid belt of millions of rocks.

[stouset]

Take out Earth, Mars, and Venus for good measure.

Whatever scattered remnants that are left of humanity hanging out in the asteroid belt are going to have a hell of a time finding food to eat or oxygen to breathe.

[perilunar]

Why?

Do you think they'll be reliant on planets for food an oxygen? That's daft — they will be growing their own food and mining their own oxygen.

Also, I doubt they will be "scattered remnants" — there's likely to eventually be a far larger population off-planet than on.

[deepsun]

Oxygen is highly toxic for most materials. I'd expect advanced civilizations to avoid oxygen.