In terms of absolute agnosticism anything can lead to anything.

Reasonable to assume that if I have a weapon that can destroy a civilization, others may have it too. Assuming also they have the similar risk calculus, it leans towards hiding and then nuking when either gets afraid enough. You can't do much but hope the other side is not more advance that you and can mitigate the attack.

It seems like a big stretch to assume aliens are going to share ideas of liberal democracy like fairness/cooperation etc (which are fairly recent) when there are groups of humans who do not

And what if they can mitigate the attack?

Then we are probably dead anyway, the onus would have been on them to contact us as they are probably several centuries more advanced. In that case its like going with a tank to an island population that still uses bows and arrows

Is it a good idea to pick a fight with a tank?

You do realise that comment supports strongly the dark forest theory right? The best thing do do before you have a tank is to shut up and hide. Of course you don't pick fights when you could get squashed

Implied was that the actions to take if we had full info are obvious. If their capabilities are known you would not have to resort to "shoot first, ask later"...

But the dark forest theory suggests when you have an arrow, you should shoot others, because they can have an arrow too. This strategy can go wrong if they have a tank. And if you systematically shoot everyone, you're systematically looking for a tank.